WinFX Developer Center Rob Relyea, in his great blog on all things Avalon/WPF, recently posted a piece on Attached Properties, a concept which makes Avalon's property system tremendously extensible.

In the article, Rob notes a community developer's lament that Attached Properties make the open-book-on-the-lap development approach all but essential. 

That'll go away when XAML AutoComplete is fully baked, I hope!