SteamOS is going to be mostly based on support of new upcoming games. If Steam will work with most of the indie dev's to push them to use openGL instead of dx I don't know, but they have been suggesting that many AAA games from now on wards should support SteamOS.... we are a long way from seeing how true that is frankly.
So realistically at best we're talking about SteamOS being like getting a new console with little to no backwards compatibility and hoping people get it for future titles almost exclusively, but with really no history of support or any idea that people will support it.
I really don't know how well it will do, I think most who try it will get bored of switching back to windows for most games. If say they threw their support behind Mantle, and released SteamOS in 2 years with a 2 year back catalogue of mantle supported games all working, with everyone knowing all future mantle games would support it..... there is more software for it from day one, and more confidence that it will continue to get support. Currently I have no idea beyond Valve games if SteamOS will really get any support at all, let alone most AAA titles.
I think most people basically will either ignore it or try it briefly, in a couple years it might be viable without having to constantly switch back to windows for all your other games.
The trouble could be how many people try it and never bother again even after two years. If it launched for the first time in 2 years with that support already in place I think it would take off pretty well, but in 2 years time everyone could have tried and given up on it, leaving it dead, who knows.
