Project Butter is the major new innovation in Android 4.1 Jelly Bean, making the entire UI fast, fluid and smooth. The entire interface runs at 60 frames per second and animations are smoother and quicker.
The other big thing is power efficiency. Jelly Bean now dials the CPU back to lower frequency when its not in use.
Jelly Bean comes with an improved Google Voice Search, now spoken by a much more human-sounding voice compared to Siri and S Voice.
The home screen has also been tweaked, adding some nice features like dynamically resizing widgets, widgets that will now automatically resize as you move them across screens.
Notifications get a major revamp with expanded Gmail notifications, multiple notification sizes, and actions to allow you to interact with apps straight from the notification display without needing to open the relevant app.
Here's a first look at Android Jelly Bean through the emulator:
http://www.youtube.com/watch?feature=player_embedded&v=iIeFxDfShmY
