Harder than it sounds, though. You still have to be able to communicate with other processes to present the user with a usable UI, so the isolation is never really complete. There are lots of variations on this theme and they all have to compromise in some way.
It's perfectly possible to have that with something like the X window system. The irony is that this protocol gives applications way too many capabilities. But Wayland is a chance to fix that for good and finally have proper compartmentalisation in the GUI.
