In order from most excited to least:
1. SSD
2. Raytracing
3. Adaptive triggers on controller
4. Haptic feedback on controller
5. Improved controller battery
6. Revamped user interface
7. Partial game downloads & installs
8. USB Type-C
9. 4K Blu-ray Player
The Revamped UI may be better than I expect but I'll have to see what they come up with because the PS4 UI is near perfect for me.