I love a lot about 4E...if I had to pick one, I'd say that there's a simplicity about a lot of it that I find very attractive in a game system.
I hate a lot about 4E...most of those things fall into a general category of things that make me feel like I'm struggling against the system to make the characters I want. This is mostly restrictions like the new multiclassing, the everything-a-paladin-does-is-a-shiny-miracle thing, the fact that only rangers are allowed to TWF, and suchlike.
(I like 4E's kukris).