The "biggie" -- multi-tasking!
When I first installed 4.2, I just kind of randomly started opening apps to make sure they would still work. (Most did!) So then when I went to check to see what was opened in the multi-tasking window (by double-tapping the Home button), EVERY app I had opened was in there meaning that it was not really closed all the way, but just sort of "hibernating" or waiting until I needed it again. To close these apps, you hold down one of them until it starts to "wiggle" (just like if you were going to move an app on your screen) and then you can close them one at a time.
I can see where it might be possible to run out of memory real fast if you forget to close out your apps periodically. I have also heard that battery life is somewhat degraded with 4.2 and this might be the reason why (but I don't know for sure).
So far.... not impressed!