Ironically, Xbox 360 seems to be doing just fine being largely incompatible with Windows games. Valve has committed to porting all their games to GNU/Linux x86/x86-64, and they have already ported a big chunk of them and other game development houses are following suit, so there is definitely going to be a lot of titles available at launch, probably more than any consoles had.
I'm sure Steambox will be as successful as the Xbox. Just as soon as they get all of the major studios to support their platform, make sure that the hardware is comparatively cheap to the Xbox and the PS3 (i.e. by selling it at a loss), and by aggressively standardizing and branding the device hardware.
They'd also need to get some exclusive AAA titles (like how Xbox has all of Fable and Halo for instance) in order to enter the market, and let's not forget that getting exclusivity on an AAA game for an entirely new console means writing the game yourself, or buying the company that makes it (Fable and Halo are both MS).
So yeah, if Steam was willing to have games that runs on Steambox but not Steam (since that would dilute the exclusivity of the titles on their console), was willing to buy other games companies and put billions of dollars into entering this market as a level-player in the console business, then yes, maybe Steambox and Xbox 360 might be a viable comparison.
But until then, not so much.