I think the most subtle-yet-potentially most valuable feature of Android 4.x is the ability to have a line of custom text on the lock screen. I flash between Gingerbread and Jelly Bean ROMs on my ol' Evo 4G, and that's the feature I miss the most, if only for the way-off chance that something happens to me (car crash, accident of some sort), someone could turn my phone on and I'd have an ICE number right there for them to look at. I know there's apps that purport to do that kind of thing already, but they either don't seem to work for me or do it nearly as simply/elegantly as the 4.x Android versions.