Lots!
but a few of my favorites:
- Electronic Image Stabilization (makes a world-of difference, especially if you are hand-holding this cam while filming)
- Much better image quality (due to a better sensor and a higher bit-rate)
- Voice Control (a feature to turn on if your gopro is ever mounted in a hard-to-reach place or if you want to have multiple GoPro control without a remote-control)
- More FOV / Resolution / FPS options