I thought I'll give a short review of the changes I noticed in 4.2.2, having used it for 1 day.
1. Previously, the "Swipe up / down to unlock" text on the stock lock screen would disappear after 3 screen unlocks, but after this update, the text will always remain on the screen. I replaced it with a
modified version of the lock screen that had the text removed.
2. The 4.2 feature that I've been eagerly waiting for is the ability to add widgets to the stock lock screen. After the update, I immediately installed
Dashclock (and its several extensions) to customize my lock screen. Very happy with this feature!
3. The notification bar and dock are both now transparent. But you'll see this only if you are using the default launcher (which I don't).
4. "Developer Options" was not available when I first accessed Settings. You'll have to go to "About Phone", scroll down to "Build number" and tap it 7 times before the "Developer Options" appear. I guess they didn't want most users to be messing around with the developer tools.
5. Previously, there was an option to take a screenshot when you long-press on the power button. That option has been removed. You can now take screenshots by holding the volume down + power buttons together. Alternatively, there is a new small app called Clip Manager that allows you to take a screenshot and immediately edit it before saving.
6. There is a new Daydream option under Settings -> Display. It's a new Android 4.2 feature, that allows you to choose what is shown on your screen (e.g. slideshow of pictures, screensaver, etc) when it's placed onto a dock. It's turned on by default, but I switched it off since I don't have a use for it. More details
here if you're interested.
7. The screen-off animation has been changed to a more subtle fade-off animation.
8. There is no lag whatsoever; the UI is smooth and everything works fast as it should.
9. I definitely noticed less heat emitting from the back camera area when using my phone.
10. You can now customize the quick settings (also known as toggles) in the notification panel. You can add new toggles and order them as per your preference. The toggles also now have labels under the icons.
11. There is a new Dualshock3 wireless controller option under Settings -> Xperia. I believe it's for you to connect a PS3 controller to the phone for gameplay. Haven't tested this out since I don't have such a controller.
12. As I mentioned before, there was an issue where the screen will automatically turn back on within a few seconds if you are using screen-off widgets like Widgetsoid. After the update, the screen remains off.
13. Battery seems to be the same as 4.1.2 to me.
14. I can't remember if you were allowed to set a custom alarm sound previously. But you can't do that after the update. You can only set custom ringtones or notification sounds.
15. The Movie app UI has changed.
16. Pictures seem to have less noise in Superior Auto mode.
That's all that I noticed so far. Overall, it's an excellent update!