The Android Developers Blog has a good overview of the preview releases. I'm linking both since they do build on each other:
O-MG, the Developer Preview of Android O is here!
What’s New in Android: O Developer Preview 2 & More
The highlights, just off the top of my head:
Good luck finding your way around the Settings menu - REARRANGE ALL THE THINGS
Press the Home button while viewing a Youtube video to activate Picture-in-Picture mode
Use AutoFill to log into apps with the passwords you've stored in Chrome
See improved security info at Settings > Security & Location
Tinker with the navbar layout in SystemUI Tuner