As I've both an iPhone and a PocketPC, I can do head-to-head comparison. I swear, it's not because I'm an Apple Fanboy, but the AppStore rocks and the WinMobApps sucks. A matter of product design, of course : the user experience per se, with the integration of the store into a global solution, the ease-of-use, etc.
For instance, the Microsoft store requires you to install Silverlight, to enable the caroussel type of product selection. Guess what : this feature is embedded into Mac OS X (ever heard of CoverFlow ?)...
That is a pretty stupid strategy from the guys in Redmond (another one ? ;-), proving that Microsoft is struggling like hell to stay... alive.
Funny MacDailyNews take on that one : "Any day now, we expect a post-liposuction Ballmer to show up at some trade show dressed in jeans and a black mock turtleneck saying "Boom!" a lot."