Is it just me, or is there a huge anti-trust issue with this concept? If you can get Metro apps only through the app store, I can see already other "store" providers like Amazon and Valve calling the courts.
And I know, Mac has an app store too, but it also had a bundled browser almost as long as Windows and no one cared.
They don't have a 90% marketshare with Metro. Only with classic desktop. You can still get classic desktop apps anywhere you like. If you don't like the Metro app store there are other markets of comparable size.