Unless Valve's plans include starting a porting company that's meant to be Linux's answer to Aspyr or Feral Interactive, the result is going to be a poorly-performing launcher/store for indie games and little else. So basically a downgrade from what Ubuntu's App Store already provides, other than Valve's own games.
And as for Windows 8... I think people are overstating how bad it's going to be for power users. The built-in app store apparently isn't even compatible with "desktop" style apps, so the Windows desktop becoming a closed platform isn't going to happen any time soon. Microsoft may be incredibly stupid, but they're also cowardly. They don't have the balls to pull off a move like that.
And as for Windows 8... I think people are overstating how bad it's going to be for power users. The built-in app store apparently isn't even compatible with "desktop" style apps, so the Windows desktop becoming a closed platform isn't going to happen any time soon. Microsoft may be incredibly stupid, but they're also cowardly. They don't have the balls to pull off a move like that.