The worst thing Ms could do is enter the app market. It creates a demand for habit. Look at the blackberry. About 1/3 of the apps in their store is from 1 dev, which can be traced back to RIM.
Well, this is a classic case of MS being caught between a roc and a hard place. They desperately need some fundamental productivity apps. But getting those depends on third party developers, who will not enter the Windows App Store until they see the potential for a return, which results in the store being sparsely populated, which puts off users etc. etc. etc.
At this rate, perhaps MS should have just worked on the Pro and worked to integrate their Phone App store with Win 8. I believe MS has something like this in the works involving RT also, but that leads a more troubling question - will future Surface RT machines be Win 8 Phone OS driven or will future Windows Phones be RT driven. I hope it is the latter.