I don't see the Windows 8 app store as a competitor to Steam though, in the same way that I don't see the Google Play store as a competitor to Steam. Steam already has a strong community knit directly into the gaming experience that I doubt Microsoft will be able to reproduce, if GFWL is any indication. And there's no way I'd pay a monthly fee like on Xbox Live. Hell, Origin isn't even that threatening to Steam, and…
I think the problem is that the Microsoft store will be the only way to get software on Windows 8 Rt. Microsoft is essentially pulling an Apple and there is no way that is a net benefit for Valve or the community in general. There are other problems with Windows 8, but I think the closed ARM version is a really big one.
Nobody is going to go out of their way to buy it, the only installs it will get is as OEM's on new hardware.
And I bet even the OEMs will offer Windows 7 for as long as they can.
I think if users _have_ to have something different they will seriously consider OSX and Linux. One is more polished on better hardware for people who have money and want the best. The other is free for people who don't care.