WP7 doesn't support HTML 5
This lack of support is on purpose, because Microsoft sees HTML 5 as a natural enemy to Silverlight and a threat to Windows in general.
Microsoft just continues to refuse to support open standards.
So, everybody is supposed to have to keep downloading large proprietary plugins, that require constant patching for security holes and arbitrary changes M$ makes whenever they feel like it. And Society at large is supposed to spend millions of frustrating hours and billions of dollars making different versions of their websites, because of Microsoft's conniving little tricks.
Further, they have announced a new DRM for Silverlight. They must think we've already forgotten about all the customers who bought DRM'ed music from Microsoft. And a few years later, Microsoft turned off their DRM servers, and customers lost the ability to play back the music they had paid for. This will make for still more incompatibility and be pure annoyance to customers.
On the Developer side, sure, MS' development tools are all very well done... but they always force you to doing things that work nowhere else.
And if someone makes .Net work elsewhere, like the mono project, soon enough, you got another lame patent fight, making it rather uncertain if you can rely on any promise of being able to ever re-use anything you've done in C# outside the Microsoft garden.
I'm no fan of Java, but at least its really portable, not fake portable.
So, I will neither buy a WP7 phone, nor will I write anything for it anytime soon.
Why would I bother... WP7 is almost as restrictive as iOS.
Google doesn't tell me what to do with Android, they don't need to bother, the stack is well enough designed. They say: here, take this thing we made, have at it in any way you like...
I can run a Python interpreter/compiler on android and code away wherever I find myself with idle time on my hands...
Only with Android does the phone really belong to the customer who bought it and paid for it...
With WP7, the phone really belongs to Microsoft.
With Symbian, the phone really belongs to Nokia and their cronies, the Telcos
With iOS, the phone belongs to Steve, he even tells you when you have replace it - after a year, when the capacity of the soldered in battery drops off.
Finally, I couldn't imagine anything worse than my phone being totally integrated with Facebook...