A preview of Android 4.3 by Pocketnow suggests huge improvements to battery-life, with the site reporting 25 hours of heavy usage.
On Android 4.2 I can typically expect 4-6 hours of use before I need to recharge. As a reminder, I use my phones quite a bit more than the average user, so my battery life is expectedly lower than what most should expect to get. On Android 4.3, without changing my usage habits, I was surprised to see the phone last all day, all evening, and still had charge enough to get me to work the next morning. I was able to eek out 25+ hours and still had 1% battery life remaining. That’s impressive! …
Other improvements noted were auto-complete for the phone dialer (offered on some carriers now but not a standard part of Android 4.2), new emoticons, and improved camera UI and responsiveness to change in orientation (which is already in the HTC One Google Play edition).
We have only a few hours left now before the official launch at 9am PT/12 noon ET.