Good idea for a thread, I hope we can keep it pretty positive in here.
1. Fixing The Math. Finally, the underlying reason why high-level play has been wonky at best, even in the edition that has done it best (3e), is addressed. No longer to you have to render one character uneffective to challenge another!
2. Monster Hierarchy. Minions, mobs, solos, elites...tremendous.
3. Flattening The Level Curve. Making 1st level PC's able to stand on their own two feet, and making 30th level PC's that are epic, but not unbearable twink machines.
4. Bloodied. A status between Alive and Dead? Brilliant. And fits FFZ's Limits system like a glove.
5. No more iteritive attacks. Oh, JOY, the flurry of whiffs is gone!
6. Less Christmas Tree. I like power to be more personal and less possession (though possession, in D&D, should still be pretty important).
7. Races affecting you at all levels. I want my dwarf fighters to be different from my half-orc fighters at 30th level.
8. Keeping, but de-emphasising, per-day abilities. Resource management is cool and all, but I want to be able to contribute something every round.
9. Defined tiers. D&D has always changed as you level up, it's good to see those specifically called out and catered to, it'll make the changes stronger.
10. Healing Surges. It's golden to not have to require a healer in the party.
Honorable Mention: D&D Insider
It's still to early to see it's promise yet, but this could be the biggest reason I play 4e. All the rules, I don't need the book, and I can play with my friends from out-of-the-state? I am tempted, sorely tempted, to love it.