Windows Phone 7's different UI gives app developers a chance to do something radical that would just be too unexpected for users on other platforms.
On the Blackberry, Apps with a lot of transitions or that on try and take advantage of big type wouldn't work out. And it certainly would look out of place with the rest of the phone experience.
On the iPhone, when apps break common style guidelines, even a little, it's noticed by users. Most of the time when apps break the guidelines they look worse off for it. The common rule is "When in doubt, do what Apple does."
One of WP7's key messages is that you can get in and do what you want more easily with their phone. And people like that message.
But what happens after you load up an app?
On the iPhone, 3rd party apps are accessed and look visually consistent with 1st party apps like Calendar, Photos, Contacts. One factor to WP7's success will be the willingness for app developers to buy into Microsoft's philosophy and their style guidelines, and to design their applications in ways that support it.
What I'm really wondering about when looking at the smartphone industry as a whole is: Do brands need to fragment their visual elements every time they want to develop an application for a new phone platform?
Do they need to match the phone's style guidelines at the expense of their own?
In that case, If the majority of applications on WP7 look and work just like their BB, iPhone, or android equivalents, than that distinguishing selling point of the platform won't mean anything for the majority of the time you spend on your phone.For app developers each new phone platform provides them an opportunity to experiment with a different workflow, and a different visual style, that works best for that situation.
But how much of their brand is lost in the process?