In the past I thought maybe it was the screen size of must WM devices that was holding back development of cool or useful app's. The iPhone had a great sizes screen that gave developers a nice canvas to work with. But now you have more and more devices that run WM that have a great sized screen. HTC as an example is putting out 4-8 devices a year and screen sizes are HUGE.
Perhaps once they open up the market place and have a central location for WM user to buy app's then can then prove to the world of developers that there is money to be made.

Only time will tell I guess.