The only reason I chose Windows over Mac OS X for my new home computer is I wanted to play games on it.
If SteamOS lets me play AAA games (and not just Portal, Half-Life, Left4Dead and Team Fortress) AND it lets me run XBMC or Plex or some other TV software, then I'd switch in a second.
I'm certainly not a "gamer" any longer, but I still have a pretty healthy Steam library.
I've been very happy with the increasing options on Mac OS X and I hope they keep improving.
I'm way more excited about slower, turn-based games, and Civ V, Europa Universalis, and FTL have been enough to keep me from searching for anything else.
Despite this, I know I've been missing out on games I would probably love. I think I see a SteamOS device in my future.
If you've already bought the machine and Windows OS then you should be able to install steam on it, buy their new controller and have a roughly similar experience with the big difference that the whole steam library will work today, without requiring streaming from another box.
SteamOS will be great for linux, but people seem to assume Valve are taking a very exclusive, traditional console approach, when they're trying something a bit different.
I also assume that basically any Steam game that comes native to Linuxt will also come to Mac OS X.
If SteamOS lets me play AAA games (and not just Portal, Half-Life, Left4Dead and Team Fortress) AND it lets me run XBMC or Plex or some other TV software, then I'd switch in a second.