Windows Phone 7 is, as we speak, losing fantastic applications because of its approval process.
I would like to quote one para from the linked post.
I can’t overestimate how frustrated my clients are with the Microsoft approval process. It ranges from sadness to fury. Clients who are on virtually all other platforms are expressing frustration that they have never had so much trouble getting through an approval process. The rejection reports are inconsistent, spotty, and fragmented. We will submit an app 5 times and get different “errors” back each time. Nine times out of ten, those errors are not errors, but complaints about UX functionality. Of those, at least half of them are complaints about functionality that, if fixed, would worsen the user experience.
I love Microsoft and I love Windows Phone 7, but rejecting apps based on the UX guidelines (while not giving us the tools to abide by the guidelines effectively) is a recipe for suicide.