I would be plenty happy if Steam would just redirect and store all the games files to a specific folder in the steam directory, much less implement a hypervisor. As it is now, installing a number of games from steam poops data all over my hard drive. I have game saves, settings and common files in My Document, "My Documents\My Games", AppData\Local, AppData\Roaming, AppData\Saved Games, steamapps\user and probably in other places I discovered yet.
While i'm at it, another nice feature would be for steam to be a unified DRM scheme. Install steam and buy a game and install only the game (and necessary libraries such as XNA for windows). Somewhere along the line I didn't pay enough attention and now I have junk like SecureROM and that Rockstar Social Club crap.