Would apps opt into this? I can see a lot of existing applications break because of this enforcement... Not that I say it's not an interesting idea... I guess there could be a lot done to make application behave nicer. But there are a lot of open questions like, e.g. how would you handle multiple users? Do you have the C:\users folders spread out over all the application directories? What about saving files... are you allowed to save files to C:\users\signed in user?