Today Google let loose Android 4.2.1 into the wild onto the Nexus 4, Nexus 10 and the Nexus 7 and Galaxy Nexus should be receiving it shortly. Now we know the main thing that was fixed in Android 4.2.1 was that December was added back into the People app. Which we found out was missing in Android 4.2. There were quite a few files touched in the Nexus 4 OTA, and thanks to Al Sutton an Android developer, we know that most of the edits were minor.
According to Al's AOSP diff script, these are the changes in Android 4.2.1 (4.2.1_r1) compared to Android 4.2 (4.2_r1). It's important we let you know these are changes in the open source code only.
JOP40C (4.2_r1) to JOP40D (4.2.1_r1) AOSP changelog
This only includes the Android Open Source Project changes and does not include any changes in any proprietary components included by Google or any hardware manufacturer. The raw log was generated using a modified version of this script written by JBQ.
0d15318 : Add HID gamepad/joystick as supported HID devices
So there are the differences in the new version of Jelly Bean. Android 4.2.1 is probably the most minor updates we've seen in a while, but due to December missing from the People app it was very necessary to be pushed out. And for many of us that are not developers, these changes probably make no sense to us as to what has changed. But this information will greatly help developers when developing for Android 4.2.1, and developing ROMs for Android 4.2.1. Now for those that haven't gotten your OTA yet, go ahead into Settings, hit About Phone, tap System Updates, and hit Check Now.
Now that we know what's in Android 4.2.1 we can get back to refreshing the Play Store for a shiny new Nexus 4. Although we are still getting errors, so good luck to those of you attempting to get one.
Source: Al Sutton