He seems like a much more conditional Liliana. +1 conditional card advantage (like Liliana), -2 conditional removal. He can probably fill a similar role in creature heavy decks.
I think the fact that Tarmogoyf has a completely unique mechanic makes it mythic worthy. No other card cares about how many card types have been played (that I am aware of).
Damage to planeswalkers from attacking creatures isn't redirected, planeswalkers are chosen to be attacked during the declare attackers step (when creatures are declared as attackers you declare if they are attacking a planeswalker just like you declare which player they attack in a multiplayer game). The damage won't be dealt to anything if they change controller mid-combat.
I think that Niv Mizzet is aware of the Eldrazi or some other multi-planar threat (via Jace) and is attempting to come up with something to get rid of them.
That's actually my biggest gripe with the card. It's not like it could have overpowered in any format at 6 mana, but it would have so much more symmetry. They really missed the mark with this one.
It's actually good as a finisher in control. It turns each card in your hand into exponential power, and is hard to remove without a sweeper. You just need 3 of them two kill them in 2 turns. Don't look at it as a gimmicky card, but as a creature that kills them on its own in a few turns.
For example: //gatherer.wizards.com/Pages/Search/Default.aspx?action=advanced&text=+[prevent">+[damage]&type=|[%22Enchantment%22]|[%22Artifact%22]"]artifacts and enchantments that prevent damage
(Damage causes loss of life, but that's not relevant here).
Edit: also, don't feel dumb about asking questions, everyone is new at some point.
That's actually my biggest gripe with the card. It's not like it could have overpowered in any format at 6 mana, but it would have so much more symmetry. They really missed the mark with this one.