The thing I miss the most is multiple user accounts.
I can't be the only one who think this should be an obvious feature for a "pc-replacement" device?
I think that a natural evolution of personal computing is for everyone to have their own personal computing device. Aside from a spouse, I wouldn't want to let anyone else use my phone / tablet / computer, as it would contain personal information. I wouldn't trust anyone else to take proper care of it either. Seems that most damage I see to phones now-a-days is from lending them to children.
For kids sharing a device, if they're old enough to legitimately need to customise theirs, and keeps their own data private, then they're old enough to pitch in for their own.