Provide mobility for user accounts (UAs) through a USB drive or the network. Ability to enable/disable this feature.
Suppose I have a perfectly customized personal UA on my computer (A), and I wish to use a friend's computer (B). Instead of, on computer B, having to:
1) use a guest account,
2) use my friend's account or
3) create a new account;
and thereby forego all the comfort and customization and everything else associated with my UA on computer A, I should be able to simply transfer my UA from computer A into, say a USB pendrive, plug it into computer B, and have it prompt me to logon to my account. Better yet, I should be able to transfer any number of user accounts I want into the USB drive, and when plugged in, computer B allows me to logon to any one of them. Syncing with computer A would be necessary in order save any changes made to the UAs while on computer B, unless the UAs are no longer specific to computer A and are simply stored on the USB drive! This is probably one key idea: having UAs installed on removable storage, independent of any one computer.
Alternatively (or additionally), using perhaps Zeroconf, computer B could detect computer A's UAs (even UAs on plugged in USB drives) and provide the option to logon to any of them. Syncing in this case might not be necessary, as any changes to the account can be saved directly on computer A through the network.
Result: Set up your user account once, and you have the ability to access it on any computer, either automatically through a LAN or with a USB pen! Wallpaper, GUI configuration, email accounts, documents and what-not, all there, anywhere! (yay, my own slogan :)