I like these things:
Character Builder for when I am making characters or asking someone else to make them. It's right there, plain and easy to create characters in minutes. The printouts making playing the game a breeze.
I like how easy it is to scale encounters on the fly if I need to, or throw something else in as an afterthought. Heck, I hardly do any DM prep work that involves stat blocks anymore, I just lay out what I think should happen and then expend the XP budget to populate.
Skill Challenges, when they run the right way. As written initially they were meh, but now that I have had time to work with it they are rather fun.
That I don't need ANY books to play at the table. If I ever need to look something up, it's usually right on my character sheet, or in the adventure I am working from.
Monster stat blocks that are easy to read, comprehend, and are not overly long makes for more time paying attention to player choices and conversations and less time with my nose in a monster manual reading about what something means that's listed as a special ability on a monster.