G+: One new feature for Android 4.2 is that …

David Coles
One new feature for Android 4.2 is that the Developer Options are hidden by default. If you want to find them you must go to Settings > About Phone/Tablet and tap the build number 7 times (should start prompting you after the first few presses).

I managed to turn on screen-update debugging while on the plane, then couldn't get back to the menu. Thankfully Tasker lets you create shortcuts to hidden activities.

Jelly Bean

(+1's) 1
Michael Poloni
What are these developer options that are hidden?

Says me who isn't yet an owner of an Android device ... 

David Coles
Mostly options like enabling USB debugging (needed for developing apps), some graphic debug options (showing overdraw, screen refreshes, blocking the main thread) and some general misc options like keeping the screen on while charging or killing apps as soon as you leave it.