Updates are rolling out to Nexus devices as we speak, but the public changelogs seem to only call for improvements to "Security." However, the latest round of commits just went up on the Android Open Source Project, bringing everything up to date with both JWR66Y (4.3_r1.1), which is going out to most Nexus devices, and JSS15Q (4.3_r2.2), which is destined for the 2013 Nexus 7. Thanks to Google's gracious sharing of the source code, we can comb through each and every little detail of what's new.
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.