Avalon's Attached Properties are a cool idea. Renaming Avalon to WPF, not as cool
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!