I think what matters most is if gamers will actually take the time to either buy a Steambox or install a Linux distro on a Windows machine. We still only have a small vocal minority, and the only way to get market penetration is to get convertees. That means an experience that is not only as good as what they get in Windows or Mac (the expectation games will install and run fine, that games crashing won't freeze up X) but needs to be better (software center integration, better performance, utilities like voice chat that compete at the top (and you can't rely on Skype)).
I see the Steambox doing really well. If it comes out before the next generation of consoles, has support for every input device type under the sun, plug and plays with a TV exceptionally well, offers a large game catalog at launch (including past Steam purchases) and has the traditional hardware sold at a loss to get user buyin, it could easily crush the Xbox 720 and PS4 if pulled off right. It would also be a dramatically easier living room system to develop for than the competitors, so I could easily see development efforts move in its direction just because of how easy it is to build a game that runs on a desktop and just putting it on the same graphics stack with consistent hardware.