Okay, so my Galaxy Nexus push-updated to Android 4.2 today.
The first thing I noticed were the visibility changes to the clock app. They made the analog home screen clock easier to see (very much needed), but then they seriously uglied up the digital clock with some font changes. You get one, you give one. The second thing was a very slight hesitation in the screen response when pressing the lock button. It's not much, but it just lost the feeling of flipping a switch. Not so good.
The third thing was the discovery of a Windows 8-style pipedream known as the lockscreen widgets. Okay, maybe some people like them, I have no objection to the feature existing. I do have three problems with how it is implemented (listed in order of: Nerd Annoyance, Nerd Rage, and Nerd Smash Phone):
1) I use a PIN to unlock, so the app list widget from the left isn't accessible unless the PIN is entered, in which case what's the point of having it? I could just log into the phone and open an app or widget.
2) I use a PIN to unlock, so the camera widget should ABSOLUTELY NOT have the ability to execute and then write data to the system memory, especially not data that might be automatically synched to another service! But it does.
3) WHERE IS THE SETTING TO DISABLE THESE WIDGETS?
Apparently, I'm not the only one, and this free, 50kB app is already available on Google Play. It requests no permissions other than, when executed, the necessary admin rights to act, and it works.
Not cool, Google.