I think if they had three compatibility VM modes: DOS/Windows 3.1 (might need a more complex DOSBox-like emulator, to solve the '64-bit OS doesn't like 16-bit stuff' problems), Win98SE, and WinXP, that would cover the vast majority of compatibility needs. Maybe there could be (downloadable from their store) more special-purpose images, for the guy who needs "WinME mode" or "Win2000 mode".
But it still feels like we're missing out on the true potential of VMs. They could be huge for privacy and security-- you run anything suspicious in a VM, and maybe only allow specifically (and user-configurable) permitted data to be persisted on disc.
I want my browser living in a seperate VM, that I can nuke from orbit in a single click. If I get drive-by downloaded, or just end up with a huge cookie file, poof, it's gone.