My personal favorites are:
1. Performance: OS X is finally usable even with transparency turned on. And many other tasks just feel noticeably faster.
2. Call out your cursor: shake the mouse/trackpad few times and the cursor will become larger for few seconds.
3. Notes app is significantly revamped.
4. Find My Friends Notification Center widget.
5. Wi-Fi calling for supported carriers (e.g., Sprint, T-Mobile).
6. Cleaner Mission Control.
7. Split-view full screen.
8. Smarter Spotlight search.