The whole fact that the apps have to be written for Metro means that all the quality software that already exists for Windows will be excluded. When you need people to write new apps, most of them initially are going to be trivial and written by amateurs because that's what's takes the least time. It's what the iPhone App Store was like to begin with too. Contrast that with the Mac App Store where existing desktop software could be submitted with very few changes and there were lots of professional useful applications from day one.