More Developer Tools
Developers targeting Android 4.2 will have more ways to profile and debug their apps while running on a Jelly Bean-based device or emulator. A new bug-report function dumps screen and state information to the local file system and kicks it off as an email attachment. There's now a way to disable the verification for apps sideloaded over USB without also undoing that security check for apps coming in from a browser. There are many more new debugging enhancements, including hardware layers that flash when updated, GPU areas that become highlighted when overdrawn, and the ability to trace OpenGL execution using Systrace or other trace tools. There's much more detailed information on the Android 4.2 API page.