I really like that 4e spells out the specific monster 'weights'. And I really like the encounter design philosophy in general. For me, those are the absolute high point of 4e.
I may be in a minority, but I like the class roles, and even that they're stated explicitly. However, I do think that the game has probably been a bit too rigid in role protection, and feel that it really hurts the game that it is so combat-focussed (in print, at any rate).
I also really like skill challenges as a concept, although the implementation is lacking.