As cool as this sounds, one of the issues i can see some people having with SteamOS is, why would they want to connect a 2nd pc device to the electrical outlet and run up their electrical bill just to be able to stream their windows/mac games, when they could just stream there games directly using windows to their t.v running just one pc. Basically more green friendly and cost effective.
Also someone mentioned earlier about other pc builders would be able to build different SteamOS specs, but wouldn't that just confuse the average consumers, they won't want to deal with having to buy the right SteamOS PC build, so that it would be able to play this so called game or have to deal with upgrading the system again just to play a newer title that requires a stronger pc. Furthermore, if there will be tons of different SteamOS builds, it won't be any different with how it is with windows, since developers would still have to program the game and optimize it, so that it would have to run effieciently across all the different builds. Where as with consoles, deveopers don't have to worry about that.