So an Xbox, basically.
Well no, not really, since consoles are devices. With SteamOS you're making a conscious decision to turn an otherwise normal PC capable of running the full gamut of PC games, including Steam games, into a box that will only run content from a single delivery system.
SteamOS isn't really for gamers, it's for Valve/Steam fans who would be quite happy to live exclusively in that ecosystem. And while there is nothing inherently wrong with that, I don't know why your average PC gamer would impose that sort of limitation on themselves. Certainly not for the sake of the nominal cost of a Windows license.
The streaming aspect of SteamOS is by far the most compelling feature for gamers who want beyond-console graphics, and that could have been packaged in an Apple TV-like box bundled with the controller and wouldn't require sacrificing your PC to Valve.