After playing with 4.2 a little, there are some interesting new features but nothing major. One interesting item is that there are new options for the lock screen. If you move the lock screen (much like switching between pages on the home screen) to the left, it brings up the camera. If you move it to the right it shows widgets you can add to the home screen: a calendar, digital clock, Gmail preview, messaging, and Sound Search.
When you pull down notifications, the settings button is now in the top right corner and, when you select it, it shows 8 boxes. These boxes are user information, brightness, settings, wifi, data usage, battery, airplane mode, and bluetooth.
As I've mentioned, developer options does not show up on the settings menu unless you enable it (by pressing "Build Options" in the About phone section seven times).
I'm not noticing much difference in speed, ymmv. I've also had problems with only one app so far; GrooveIP will not sign into Google Voice. Per the GrooveIP people, Google changed on of the libraries GrooveIP used for encryption, they hope to have an update out in a couple of days.