You almost have to have a dedicated XAML/UX guy on staff for even the most trial of applications. I've also had some performance fun with UIAutomation performance related issues. (UIAutomation is the snooping Windows technology that let's apps like screen readers, user accessibility apps, and things around touch snoop into your app) UIAutomation has bugs that can also slow your app down. You don't have any control over it other than writing some hackery to try and block it from peering into your app and in the process preventing those with accessibility issues from using your app. This has been a problem since .Net 3.5 and wasn't fixed in 4.0. Here's one flavor of it http://social.msdn.microsoft.com/Forums/en-NZ/windowsaccessibilityandautomation/thread/6c4465e2-207c-4277-a67f-e0f55eff0110. I can make my app's combobox drop down render time go from sub seconds to minutes just by plugging in my digitizer.
Oh and don't get me started on the simplicity of the UI thread, dispatcher, et all... I'm all for a fluid user interface but displaying the hour glass in a WPF/MVVM application should be so damn expensive to code.
Don't get me wrong: I beleive WPF is the right idea but it's way to expensive to use to develop most business applications.