This is more developer related, but one of the biggest news is a backwards compatible ActionBar in the official support library. All that's left to say is a big, enormous thank you to Jake Wharton for keeping us all sane until now.
Seriously, this might be the most important change in the list. Jake's work has been incredible, but having this provided by the support library is a huge improvement to Android development.
Um, its worth nothing that ActionBarCompat is not an addition to 4.3. Its an addition to the support library. If you include the support library in your project and you can use features from 4.3, such as fragments, and the action bar all the way back to android 1.6. The support library is a backport of modern functionality.
This is part of googles approach to addressing fragmentation on the Android platform.
Small correction, the support library which has ActionBarCompat only goes back to 2.1 (each version has a number representing the API version it supports, in this case v7)
Edit: The video about the ActionBarCompat is available: https://www.youtube.com/watch?v=6TGgYqfJnyc#at=20
Seems largely identical to ABS in its implementation, which makes sense.