I probably forgot a few, but there are just so many things I like about this game.
Skill Challenges: I love it as a framework because it has helped me decide if the players "made it" or not. Pre-SC's, my decisions were sometimes a bit too arbitrary.
Rituals: I like that utility spells are no longer the province of the wizard, all while he remains of of the best to cast them. I love how many spells have been removed from combat, to avoid certain issues from prior editions. There is still room for improvement I feel. Rituals should probably have been tied to surges instead of money and instead maybe the casting time should have been taken down to 1-2 minutes.
Fixed HP progression: Freaking hell, I love this, simply because it removes all the whining from players that constantly complain about their sucky hitpoints. Also it removes an additional randomness factor
DM Tools: While the CB is nice and all, the MB is just an amazing tool for any DM.
The DM/player division, book-wise
Monsters and players not made from the same set of rules
A fraction of the prep-time
I love the split between at-will, encounter and daily powers and the focus on class balance. I would still have preferred if the classes had been more diverse.
Minions - because how else do you make a fun fight against 45+ enemies?
Solos and elites - because the game nees awesome monsters without having to do a gazillion damage.
Monster roles - because they help DMs analyze the dynamics of his encounter andimprove them
The flattened power-curve - because spikes are never good
Attack vs Defenses and One "BAB" progression - so much easier and consistent
PoL - because it's quickly becoming one of my favorite official campaign settings, bypassing old friends like Greyhawk and Forgotten Realms.
The use of other stats than str to determine melee attacks
Healing surges! - best thing since sliced bread
(Save ends) system, makes tracking durations and effects much easier. And recharge mechanic for monsters.
And last but not least:
I am having the most fun as a DM since I started DM'ing, almost 20 years ago.