It would have about as lousy app compat as the Kindle Fire, which is to say nothing like as much as Google would like you to believe.
I'll say it: the Amazon APIs are actually better than some of the Google equivalents too. MS can play exactly the same game, but with the muscle of small business users.
Windows Phone simply has negligible actual interest from the dev community because it's too much of a technical island, and tied to the whole Metro disaster.
