...can play all the games in the Steam store, all the games in Origin, AND all the games in the Windows 8 store.
SteamOS players still need a Windows PC to play the vast majority of games.
So why bother with SteamOS? Cut to the chase and plug your PC into your telly, or get an XBox or PS4.
DOSBox and WINE aren't a selling point; if you're buying something like a SteamBox, that shit needs to work out of the box and without the customer even knowing it's there.
Newell has seen what's coming down the pipe, and it's basically his proprietary, DRMed-to-the-nines, walled-garden being rendered obsolete. He's using Linux because it's cheap, not because it's "open". If you want to find out how "open" Valve is, ask 'em for the Steam source code.
(The controller does not "make all other controllers look like shite", and Half Life 3, if it even exists, which I don't think it does, will come out on Windows first, I'd bet a pint on that. Gabe likes to blather on about linux, but he likes money more.)