Am I the only one who remembers that story, where Gabe Newell himself told how linux was actually better performance wise for pc games? I remember that he showed Left 4 dead as an example, where on linux they got 20-40% more fps than on windows. They should push that story if they really want more people on their side.
I think that Windows as an OS is spread a bit too thin for all it's purposes. Think about it, it's used everywhere, from offices to servers, and probably because of this, gaming on Windows is the last thing on Microsoft's agenda, which is why they so desperately are pushing the XBONE. OK, let them have it, let windows be used for work, I myself wouldn't have it any other way. But it's about time someone actually took the initiative on making truly dedicated software for gaming, since the PC gaming hardware is ever evolving, and people actually are buying their TITANs and i7 processors, which, so it seems, are already better than the hardware in PS4 and XBONE, not to mention the WiiU.
Long story short, i think it's obvious why AAA devs favor the consoles, it's because of dedicated software, and finally Valve is offering the same dedicated framework, only on pc, where anyone can make their own hardware setup, depending on their financial capabilities, and currently available hardware.
Just think about it, PS3 had about 512 mb of RAM, and for the same games,(say, for example, Fallout 3) to be played on Windows, you needed at least 2GB of RAM. Crazy ain't it?