Users are fickle, but this play was tried by EA with Origin. You can get them onboard with the big titles like Battlefield 3, but they've been less successful at getting users to buy titles from smaller publishers. Steam offers a better deal to small publishers, and is actively engaged in the community in a way that I doubt Microsoft can replicate.
As for Xbox-PC game interoperability, that would be interesting. I know multiplayer between the two platforms has been ruled out, but transferring from one to another might be attractive. It also makes you wonder why they haven't done it. Games built for the XBLA are written in XNA and can be played with minimal adjustment on Windows. But practically every XBLA-PC success I can think of (Bastion, Orcs Must Die, Limbo, etc.) has been put onto Steam. Makes you wonder why Microsoft isn't already mandating XBLA games be ported into Games for Windows Live if they go to PC.