Silverlight layout performance
The biggest problem I almost exclusively run into when working on WP7 apps is the layout performance of Silverlight. It's takes quite some effort to work around that problem. And according to web searches about this problem I felt that I'm not a lone with this. It'd be great if you would work on a solution to improve the situation.
They won't fix this.
If it comes to performance, WPF and Silverlight are not the tool you are looking for.