Mechanic Rundown
Great Mechanics:
Cycling; this is the holy grail of mechanics. Not only can you play it in any deck, without caring about whether or not you have other cycling cards, but it's a powerful mechanic that isn't absurdly power or forces drastic cost increases. Eternal Dragon was reasonably costed at his time, even if he didn't have Cycling, but Cycling also didn't make him broken, it just made him a bit better.
More so, you can build around Cycling if you wanted to as it's not only a decent mechanic but can be used as an engine for decks like Crypt or Open the Vaults where you're using Cycling as a main theme to power cards that aren't directly created to make Cycling better (as in Slide). So in short it adds a lot to the game without taking anything away.
Dredge: It's a bit more restrictive than Cycling because of how much stronger it can be, but it still interacts with mechanics like Flashback, can be built around, or simply used as a stand alone card. You also don't need to use it in a Dredge deck, because as long as it's reasonably costed it adds to the card and there are cards you'd play regardless if they had Dredge.
I could play Modlervine Cloak and Call of the Herd in so many different decks without having to depend on one card or the other, but both cards made each other better.
====================
Good Mechanics:
Flash Back: Good mechanic that goes into any deck, but stands above the other good mechanics simply because of how it can be costed in order for it to interact with other mechanics and deck types. In short things like Therapy and Dread Return makes it a bit better than other Good mechanics. It's also nice with Looter abilities.
Scry/Wither/Kicker/etc: Strong mechanics that go on any card really (Wither is the tricky one but in theory it can go on an enchantment/artifact if needed) and don't force your deck in one direction or the other. However, unlike the great mechanics you can't really figure out innovative ways to abuse them and combo them out unless there are cards printed to specifically support them (example; Rumbling Aftershocks).
=======================
Ok-ish Mechanics:
Metal Craft/Landfall/etc: People will argue this but in all honesty while these mechanics are restrictive you can still play them without too much worry. Almost all decks play Lands so Landfall isn't too terribly restrictive and can do various things, but it does promote a certain kind of deck and requires things like Fetches to be printed.
Metalcraft is worse than Landfall but all of the cards are still playable on their own and you have room for things like Galvanic Blast which is an ok spell that sometimes does something more. As in, it's Shock plus something, instead of "these cards can't be played unless you're supporting heavy artifacts".
Affinity: It's borderline bad because it requires you to play heavy artifacts or your spells are overcosted usually, however it doesn't always force you into artifacts as some spells aren't "useless" if you're not in artifacts... Frogmite is still a Grey Ogre in reality as Shadowmourne shows us that 1 coloured symbol equals roughly 2 colourless, so 4 for a 2/2 is kind of like 2R for a 2/2.
However, this is where we see a problem showing up, the mechanic is starting to be so specific that it will almost always either be overpowered or underpowerd. Remember now, a good or bad mechanic is different than a strong or week mechanic in terms of play. Being bad "for" Standard isn't the same as being bad "in" Standard. Clearly with Affinity we saw such a strong push in the form of Artifact lands that the mechanic was busted in half, but when you removed artifact lands it became "ok" at best.
=====================
Bad Mechanics:
Infect: This mechanic is the very definition of being restrictive and is in fact a good mechanic subsidising a bad mechanic, that was made worse.
It forces you to play Infect, without really allowing you to play the cards easily as stand alone. Sure you can use Phyrexian Crusader as a blocker in UB but that's because of the card itself and Infect only serves to make it worse where Wither would make it better.
The mechanic forces other mechanics and cards to be printed in the set that almost specifically support it. Proliferate isn't a very strong mechanic and is mostly around because of Infect, however, even then people don't like using it for Infect but rather for other cards that use counters (Walkers, Ascensions, Etc).
And finally we will either see Infect never being good enough, or being too good (as in the case of Affinity), in the future simply because of how much support it requires to make it playable in the first place.
So in short, it adds nothing to the game, restricts how you build decks greatly, doesn't interact with any other mechanic aside from ones created to specifically support it, and can be replaced by other mechanics in the long run. If anything the mechanic is used to make cards weaker, as in the case of Phyrexian Crusader who would be so much stronger if he only had the Wither part, to the extent that he could be a little too strong almost, but instead he has Infect and turns into a sub-par card.
Not only does it initially fragment it, but as you stated it forces mechanics to support it if it's to be any serious mechanic, thus fragmenting the game further... Until eventually we have two different games that don't even really make sense together.
As silly as it sounds, I could play Magic vs. Yu-gi-oh and come out even, as long as we keep our life totals respective to each game, but removal equal. This isn't to compare Magic to Yu-Gi-Oh, but rather to say that making two games out of one, but dividing simple things like types of damage done by creatures is not a good idea. Sure your creatures do X times more damage, but my life total is X times more as well, or in this case my life total is X times less and your creatures do X times less...
You bringing up things like tribal, or mechanic support, shows me that you don't grasp the point I'm bringing across. Infect creates a secondary game within magic, not a mechanic alone, and that's the issue.
Tribal was playable with other cards, and didn't matter if it was Tribal. A Tarfire is still a Shock, that just happens to have extra rules text, but a card with Infect isn't the same as a card without Infect when needed to be. Imagine if Tribal cards did a specific kind of damage. That Tarfire now does "Goblin Damage" and your opponent has a regular life total and a "Goblin life total".
Does that help explain things? Distance yourself from the fact that Infect is linear, and focus more on the part where you simply can't play Infect and Non-Infect cards in the same deck as you'd be focusing on two life totals during the game instead of just one... There honestly isn't a comparison for Infect as a mechanic, because there hasn't been one in Magic's history that has literally kept you from playing with other cards as a whole in such a visible way.
Taking this to a bit more of an extreme, imagine if Wizards said you can only play a maximum of two colours in your deck, period. Infect is a softer version of saying you can play "A creatures/spells" or "B creature/spells", where as every other mechanic says "Preferably play with more A or B cards, but you can still play them with any other cards."
I agree in that Infect is an exclusive mechanic. It does encourage playing almost excusively with Infect. However I would like to make the counter point that Infect is really the first of its kind. It is the first Poison mechanic to actually have any major effect, and as such will be very exclusive. You could make the same point about tribal when Lorwyn came out, if there were absolutly no tribal cards prior to Lorwyn. My point is really that Infect occupies its own design space, with absolutly no connected mechanics. So yes, judge it. But do not throw it away out of hand.
As I was saying above, I do find things like Slivers boring, but I'm not exactly going to call them bad mechanics. Slivers can be played in non-Sliver decks which is the important part, and we've seen various Slivers pop up alone in various decks.
Infect, because of the secondary life total basically doesn't give you that option. You can't play 1-2 Infect creatures in your regular deck, as they end up making your deck inconsistent unless you focus on them specifically. There's no fun in finishing the game with your opponent at 2 life and 8 Poison counters while you're at 0 life.
Furthermore, it restricts what cards might see play as a whole in various formats. Phyrexian Crusader could have easily made a splash in Extended in various decks if he had Wither as opposed to Infect, but as it stands now you can't really play him in your Jund, Doran, etc builds, simply because he has Infect. Something being a Sliver has never stopped it from seeing play in various decks, but with Infect that's clearly the case.
So as I said, Infect isn't simply linear, it's exclusive to itself, meaning that you aren't simply compelled to play it in mass because of how it is, but rather you have no choice but to either play it in mass or not at all when it comes to constructed.
=====================
Remember, don't think about the cards as much as the mechanic itself in an abstract sense, because I'm not discussing the power of a mechanic, rather the usages of it.
Now imagine if Wizards announced tomorrow that all creatures in the next set would be divided into "A" creatures and "B" creatures, and each player would have a different life total for "A" and "B" damage. What does this mean to you?
To me it means they're adding bloat to the game that literally restricts your ability to play cards of one type or another, as you can't be focusing on an "A" life total and a "B" life total without hampering your deck greatly. This is essentially what Infect is, it's a segregation of which creatures you can play with which other creatures.
This isn't the same as a tribal theme or whatever else, this is simply a segregation, because whether something is a Goblin, Sliver, or Contraption, it still deals "A" damage, like every other spell and creature. While on the other hand Infect deals "B" damage and just so happens to have Wither on all of its creatures.
See the difference between a linear and an exclusive mechanic?
Rant aside, I agree Infect is a very linear mechanic and there aren't many cards to play with, but I also say that there are more on the horizon. I know this point has been made before, but you can't dismiss Infect till we have seen its complete cycle of existance. If you were writting this after the release of Besieged, I'd have less of a beef. But you cannot dispell Infect simply because of its linearity.
I completly agree with Persist. It is basically flashback for creatures, albeit with a way to get them to come back. I love Persist, and I hope to get my hands on a couple of boxes of Shadowmoor/Eventide to round out my collection. That being said, it does mess with the game and the right deck can manage to get an infinate mass of creatures. This is hard however, as the correct tools aren't really availible. This is one of the simplest, but not neccessarily the best ways to balance a mechanic. I really dislike this methodology however, and really wish they would have (or do in the future) print more persist/Persist support, and balance it by putting the mechanic in the appropriate enviornment.
I'm sure I neglected to mention tons of mechanics, as this is mostly an explanation about why I think Infect is a bad mechanic, and as such I just listed a few off of the top of my head that I thought were general enough to get the point across.
I'd say most mechanics fall in the "Ok-ish" to "Good" range as it's rather hard to make a mechanic that simply cuts a colour apart like Infect does.
Though I would put persist up there behind Dredge and Cycling, it was strong with out being forced and set up a lot of cool interactions with +1/+1 counters and -1/-1 counters.
You could just play good persist creatures or do stuff with agani goldmane etc.
That being said I guess I should give a little prelude to this whole post. Basically this was a post I made in the Mirran/Phyrexian Crusader thread when I got tired of people repeating the same old phrase that "Infect hasn't been fully revealed yet, it can still be good in constructed!" when I said "Infect is bad for constructed". The difference being that a mechanic can be good in various formats but it doesn't mean it's good for the formats, as in it doesn't really contribute anything to them aside from itself and has little to no interaction with everything else. So this isn't about cards themselves as much as the general theory of various mechanics and how I view them in terms of what they can provide for a constructed format, and specifically Standard.
My issue with Infect isn't that it's very obvious and straight forward, as I don't mind Allies or Slivers, aside from the fact that they tend to be boring, but rather that it has no interaction and forces you to pretty much only play Infect. As in, Allies are obviously supposed to go with Allies but I can still play a Battlemaster as a 2/2 for 2 with various abilities in decks that don't really run other allies... Silvers are the same way, Harmonic Sliver is a solid utility creature first and a Sliver second. Infect however doesn't have this benefit as we can see from Phyrexian Crusader who instead of being a beastly aggro card becomes only playable in Infect and maybe as a sideboard card in UB to act as a wall, but because of Infect it can't really be an option as a maindeck card like it would be if it simply had Wither. In short, Infect doesn't only work as an overly simplified mechanic in terms of what it works well with, but it essentially cuts your creature options as you can't really focus on two different life totals. On top of all of this, once it's there, it's there, and you don't have ways of really doing much about it once it happens, thus making it a very onesided mechanic... It's just a pile of bad stuff.
Moving on, some one else brought up the Dredge issue, and while I agree that it's a controversial listing, but I think my reply to him is valid for this too:
I see your issue with Cycling, but what you have to understand other colours have better options than simply playing Cycling on its own. As things like Stoneforge, Hawk, Visionary, and so forth all offer up card advantage without too big of a tempo hit, while Cycling offers a minor tempo hit in exchange for a cantrip and nothing more. Blue still has the advantage because things like Ponder or Preordain aren't simply cantrips but allow you to dig or set up your draws.
Finally Cycling is generally printed in such a way that it's not simply superior to other cards of the same type, rather it's a "sidegrade" that opens up new options. The lands with Cycling weren't simply a land with Cycling but a land that came into play tapped and was non-basic, meaning that you had to make a choice of how much tempo you were willing to possibly give up in terms of playing these lands and how many of them you truly wanted... So really, Cycling makes slightly underpowered cards playable, while also opening up new tactics simply because of how many things Cycling actually does without most people realising it. Open the Vaults wasn't an incredibly broken deck despite having a huge amount of Cycling and not too much counters in the format, but it still used the discarding aspect of Cycling and the Cantrip to fuel its graveyard...
============
Wither actually saw print on non-creature spells, such as Puncture Blast.
The reason I lump those mechanics together is that they're good but they aren't as interesting as Dredge or Cycling in terms of play. Of course, like everything they all need to be done in moderation, but I'm not opposed to seeing any of them showing up, although I'm not looking for ways to break them either when they do show up.
============================
Landfall existed before Landfall the keyword, it came out in Ravica in the form of a Treespeaker and Vinelasher. It can be interesting but at the same time it's a lot more restrictive than the simplistic mechanics mentioned above and it doesn't really allow as much interaction as Cycling and Dredge. It also pushes you into a certain direction more than Kicker, but not as much as to force you into a purely landfall deck.
===========
Affinity was a mechanic I liked, but mostly because Affinity was an Aggro-Combo deck that was incredibly interesting. However, as a whole Affinity was broken by the fact that Wizards wanted it to be playable and gave it a little too much support in one block. The artifact lands were actually the biggest problem in Affinity as not only did they fuel Ravager and Disciple but they basically produced 2 mana each for cards with Affinity. Cards like Forgmite, Enforcer, and Thought Cast aren't as good when your lands aren't producing 1U or 1R each for them and your Chrome Mox is also producing 2 mana.
Of course Affinity could still be ok if not for Disciple, Blast, and Ravager, but then it would be a little underpowered as a whole.
===================
Boy did I have a lot to say on this, maybe I should start submitting articles and calling them something silly like "Mana Myr Monday Material" xD.
Cycling is a fine mechanic. Everyone loves drawing cards, and almost every deck is improved by having a couple of decent cyclers. The big problem with cycling is that it fundamentaly changes the way the game works. If everyone is drawing cards more effeciently, then the game gets accelerated. However lands are not being played any faster (assuming your using a single block which has cycling), so player's access to mana doesn't really shift. This hurts blue, one of whose primary attractors is amazingly efficient card-drawing spells. Cycling is a good, straight-forward mechanic for players, but a headache for designers.
Dredge is one of the worst mechanics in the game. It is also one of the most fun mechanics in the game. The player who intelligently uses Dredge in their deck design will almost invariably end up with a powerful deck, with plenty of options. However the player who isn't playing dredge loses game time, as they have so many fewer relative options, dragging the game out. Dredge also messes with two of the balancing elements of the game: the random draw and the graveyard. The random draw is what makes Magic work, as it stops any one deck from automatically winning, as a player doesn't neccessarily draw everything they need. The graveyard is where dead cards go to die, and should, for the majority of the game, remain untouched. Of course there are obvious exceptions to these rules, as there should be. The problem is that Dredge breaks both of these rules, without much cost to the dredge player. I personally love dredge, but I also think it is one of the worst mechanics for the game as a whole.
Flashback almost falls into the category of Dredge, but doesn't due to the exile clause and the generally high mana cost for the effect. I'll be honest, I don't have that much experience with flashback, but from what I have played with it, it seems to be a fine mechanic.
Scry is a simple way to allow a player to alter the random element of the card draw, without overtly changing the shape of their library. One of the better mechanics I have seem, its primary weakness is wordiness, and an expodential power level as the Scry # gets higher and higher.
Wither is a strange creature mechanic. I call it strange in that it doesn't really effect how my creature works, rather how your creature will work in the future. Due to this, there is very little design space within the mechanic. This is really its only weakness, as it is hard to break such an obviously abusable mechanic.
Kicker is an option giver. That is all it does. Period. This makes it, in my opinion, among the best mechanics ever printed. It adds options to one player, without detracting from the balancing elements of the game (like dredge or cycling) or taking options away from your opponents.
Metalcraft is Affinity V 2. That's all it was designed to be, that's all I expect it ever will be. Overall a limited/casual mechanic, I have no issue with a power pumper like this. One of several of the block mechanics, it encourages a certain theme in a set.
Landfall. Another block mechanic, landfall is simply a way to push the importance of lands without fundamentaly changing the lands. Personally one of my favourites, I'd rank Landfall up for originallity and uniqueness.
Affinity is another one of the worst mechanics in the game, and another of my favourites. While Dredge broke card drawing and the graveyard, Affinity breaks mana. Without the balancing effect of mana, the game breaks down into a "I win, you lose" scenario, without an opportunity for the loser to respond. And it wasn't the artifact lands that broke affinity, it was affinity that broke affinity. Anything which fundamentaly alters one of the game's balancing aspects on a large scale is a bad mechanic.
Infect is a block mechanic. Nothing more. It is meant to sell Scars and to be a powerful casual/limited mechanic. Your primary beef with it seems to be that it is linear, but I would disagree and say that this is a good thing. For a set to be successful, it should have at least one or two linear mechanics which players can immediatly lock onto and play with. One of the reasons Ravnica was so successful was that each Guild was very well defined. Sure, players played multiple guilds in limited. But they almost always worked best with other cards of the same guild.
Just some thoughts. By the way, what was with the etc? It is not immediatly apparent what you are talking about there.