Like: I only get to pick one thing? That's a tough call... but I will go with bounded accuracy and all that it implies (removing expectations of magic items, strictly limiting bonus stacking, et cetera). In addition to being a good thing in itself, bounded accuracy helps enforce simplicity on the system.
Dislike: The skill system, which is the one place bounded accuracy causes problems. In combat, the bounded values (attack bonus, save bonuses, AC, save DCs) represent only part of your total combat power; you also have number of attacks, hit points, spell slots, and so forth. These unbounded values can scale as much as needed to make sure that specialist PCs shine in their specialties.
With skills, however, the bonus and the DC are the only values involved. As a result, the difference between a skilled PC and an unskilled one is hard to notice at low- to mid-levels. I would really like to see a rethinking of skills, to incorporate a non-bounded element of some kind.