So, you want to know about everything that's new in Android 4.3. Every single change that's occurred since 4.2.2. Well, if you don't mind scrolling through a barely-human-readable, mile-long developer changelog, now you can satisfy that urge.
Al Sutton has posted a wonderfully-formatted AOSP commit changelog covering the changes from Android 4.2.2 (JDQ39) to Android 4.3_r2.1 (JSS15J). The JSS15J build is the most recent release of 4.3, and is the one currently shipping on the new Nexus 7. Older Nexus devices are currently on slightly older builds (prefixed by "JW") in order to protect them from destabilizations that might have been caused by last-minute changes to AOSP specifically for new N7.
We've published these changelogs as embedded elements in the past, but Al's done such a great job formatting and cleaning up things that there's really not a point, so be sure to head to his page to check it out. Who knows what could be lurking in there!
You can check out our previous developer changelogs, too - Android 4.2.2 is here, and Android 4.1.2 is here. Finally, if developer changelogs aren't exactly your thing, check out these posts discussing in a more general sense 'what's new in Android 4.3?':
- A closer look at the major changes to Android 4.3, generally.
- Multi-user mode now allows you to restrict apps or settings on profiles.
- The notification listener service lets apps control the notification bar more directly.
- Root is kind of broken at the moment.
- Photo spheres should look a lot better.
- There's a hidden app "permission manager" - aka App Ops.
You can also watch the Google event at which Android 4.3 and the new Nexus 7 were unveiled right here.