I think a lot of people are expecting all of their Steam games to run on SteamOS or a Steam Machine. That's not going to happen for a lot of Windows DirectX games. Games that are DirectX 9 can be emulated with Wine (it's not an emulator, yeah, whatever), but I don't think Wine supports DX10 or 11. If Valve hasn't solved this problem, or secured a DirectX license (unlikely), you're left with the same options you currently have for Linux gaming -- you can play OpenGL games or DX9 Windows games through Wine.