I don't know ... An OS that can't run Battlefield, or Bioware RPGs, or whatever else EA has that's good. On top of that, Blizzard. I know DOTA is huge, but why play DOTA in Steam OS when you could play it in Windows and have all those EA, Blizzard games available without dual booting?
Edit:
If they sell a dirt-cheap streaming client for people that already have PCs, then I could see that being a success as a peripheral, for those that want to game on a second screen in the livingroom or whatever. Would I buy a $500-1000 Steam OS PC? Probably not, unless for some reason the hardware is really good value for money, but I'd still end up putting Windows on it.
Don't forget League of Legends.