I've only had it a few hours, but I've already discovered two things I love...
Safari: I can now navigate to a web site or start a search from a single field. Google Chrome has always been that way and it's so much a part of how I work, I had pretty much given up on Safari. That one feature makes me more likely to make Safari my default browser again.
Notifications: Love it. Entourage's My Day app has been banished from my dock.
Bonus: Immediately after the upgrade, I got an update that fixed the retina display glitch I'd been experiencing.
Can't wait to find out about other improvements lurking in the shadows.