A level 14 half-Drow Paladin from Icewind Dale, Aric Annea rover at Amazon's End User blog has put together a list of three features Microsoft could introduce to Windows 7 to reverse Vista's Charybdis-like coriolis of suckage.
Given Vista's problems, three changes seems like a pretty forgiving list, but they're good ones. I like Annear's suggestion of allowing users to easily boot into multiple environments and virtualize between them as a bridge between necessary kernel updates and backwards compatibility, but this is probably the one that hits most close to home: the ability to buy core operating system additions through a Steam-like delivery service.
I still can't think of a defensible reason to sell multiple versions of the same operating system that vary as slightly as the versions of Vista do. The two steps above would eliminate the need to sell multiple editions: you buy the base kernel and environment, and extend it via the Live-derived software package and update manager to become the version you need. You could even sell additional microkernel environments like Micrsoft Server or Media Center, since they'd install separately and simply be an addition to the master boot record. The key caution of this approach is not to nickel and dime the customers to death in trying to meet their basic computing needs, but the core concept is sound.
Nickel-and-dime micro-transactions are indeed the risk here, but as I wrestle with a buggy German Vista install hacked to get around Microsoft's bizarre "only one language outside of Ultimate" limitation, the ability to simply log-in, pay a couple bucks for the English language pack and go on my merry way would be heart-achingly welcome.
Three Things We Want To See In Windows 7 [End User Blog]