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.
Why the split? JBQ posted the reasoning on the Android development forums, basically clarifying that a separate branch of the code was created to remove the risk that last-minute changes to code for the 2013 Nexus 7 would destabilize existing devices. Once things settle down a little bit, these code bases will be merged, probably not long before the same steps are taken for upcoming Nexus devices.
Al Sutton has again provided cleanly formatted versions of the changelogs for both JWR66Y and JSS15Q. Of course, you can also check out the previous, and quite massive, Android 4.3 changelog. As Al notes, these may be incomplete releases, suggesting some changes may have been held back.
Let's start with the New Nexus 7 and its issue with erratic multi-touch. As you may have already heard the latest update supposedly fixes the problem, and it appears we can chalk this one up to updated touch firmware in the kernel.
Remember App Ops, the new tool for controlling app permissions at a granular level? There is quite a bit of activity in the code, including the removal and re-addition of App Ops because it is "Not yet ready to expose." One interesting tweak is that changes made in App Opps will be cleared out when resetting application preferences. So when you decide to reset your default browser, you will also be clearing out your custom settings for app permissions.
Aside from what's listed, everything else looks fairly uninteresting. There are fixes to networking code, handling for bad storage, and a few other little tweaks here and there, but nothing to get very excited about. Don't worry if specific bugs you may be experiencing haven't been addressed with this OTA; 4.3.1 can't be too far off, and it will surely bring some more significant fixes.