MTG Salvation

For Reference:

curiosity, Keen sense

There's a lot spoiled (for better or worse) in this card. Compare the text of Keen sense to Sixth Sense:
Whenever enchanted creature deals damage to an opponent, you may draw a card.
VS
Enchanted creature has "Whenever this creature deals combat damage to a player, you may draw a card."

1. WOTC wanted to keyword a curiosity effect. We can tell because WOTC jumped through hoops to give the enchanted creature the ability. See spirit link vs Lifelink; spirit link, like keen sense referred to the aura's controller; so you could use it as a bad pacifism effect if you wanted to. If WOTC wanted to avoid this, and make it only benefit the creature's controller, they would have worded it:
Whenever enchanted creature deals damage to an opponent, it's controller may draw a card.

2. There is (or was) a card in Amonkhet that would be too good if they printed keen sense. Either this is because of Muraganda Petroglyphs and Sixth Sense is a way to shut off that +2/+2... or, more likely, it's a 2 card combo for winning the game with non-combat damage.

This likely prompted the change from damage to combat damage. Note that from a design perspective, this would be quite a mistake - it would be like making haste read "(This creature can attack as soon as it comes under your control.)" instead of "(This creature can attack and t as soon as it comes under your control.)", shutting off hasty mana producers, etc. WOTC might want to make a curiosityUR tim. Thus, once the 2-card combo was discovered, they had to nerf the enchantment, and rather than nerf the keyword, they simply removed it.

3. Amonkhet likely has a few (now likely unkeyworded) curiosity creatures, likely in UG, but there might be a B splash (think replacement for skulk).

4. Amonkhet was designed poorly and/or rushed.
There is a substantial complexity and cost difference between:
Enchanted creature has "Whenever this creature deals combat damage to a player, you may draw a card."
and
Whenever enchanted creature deals combat damage to an opponent, it's controller may draw a card.

George Lucas is said to have changed Revenge of the Jedi to Return of the Jedi to save on printing. More importantly, when constructing a card game, linguistic simplicity is desirable. The second wording is objectively better, which suggests the more complex wording was either an artifact of a last-minute change (limiting keen sense to combat damage and removing the keyword) or poor design... or both.

Another indicator here is that Sixth Sense sounds like a great name for a card; it's simple and evocative. It's an ideal name for a card you'd want to reprint. If Sixth Sense was going to be the next Lifelink, it was named well. But now it'll pretty much never get reprinted, so it's a waste of a good, simple name.

Finally, it's a new card that is needlessly strictly inferior to a pre-existing original - and original that wasn't highly competitive, but was somewhat fun around the kitchen table. This isn't the sort of card you want to print. If Sixth Sense had given +1/+1 (and maybe cost 1 more); or had Flash, or something else, there's little chance it would be significantly better, not-strictly worse than keen sense, and MAYBE playable in some format somewhere.

There can be some sensible disagreement about Amonkhet's mechanics (discard/cycling, -1/-1 counters, exert, embalm -> unique token), and I think that even the most pessimistic magic player can find something to love about them and the flavor (but perhaps not the card design for the masterpieces?); but Sixth Sense suggests quite a lot about Amonkhet and its design team. I like that they wanted to keyword curiosity, but I hate they decided to nerf a combo by nerfing a keyword, then unkeywording it. But I don't like that the final product is this... sloppy. It's amateur design - and it's quite stunning that no one caught it.