@evildictaitor: You mean I can make them overlapped at any size, like we've been able to do for the last 20 years? If you can tell me how I'll give you a dollar.
One nice feature of Metro's thirds is that other Windows then respect the fact that you have something that shouldn't be overlapped and maximise accordingly.
This means you can, for example, have, say Windows Live Messenger in one third, and use your desktop in the other two thirds. No matter what you do to your desktop, nobody will ever pop their window over or under your metro third, and you can still Aero-snap on your desktop to maximise (to two-thirds screen) any app you're running.