Android 4.2.1 along with its source were released today, but outside of the December bug in the People app, it wasn't immediately apparent whether it contained other fixes and improvements or not. The list of files touched by the Nexus 4 OTA was extensive, but now thanks to developer Al Sutton, we can confirm that most of those were probably just minor edits to bump the version number.
According to Al and his handy AOSP diff script, here are the only changes in Android 4.2.1 (4.2.1_r1) open source code compared to Android 4.2 (4.2_r1). Note: There could be other changes in closed-source components.
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
And there you have it - Android 4.2.1 is probably one of the most minor, yet necessary, updates ever pushed out by Google in a point release. Another mystery solved, now we can concentrate on finally getting that Nexus 4 ordered, this time, hopefully, without Google's servers melting down.
Source: Al Sutton