This is the first list I’ve seen that covers the new features being added to WPF in .NET 3.5. Top three things that stand out to me:

1. Binding support for LINQ based data sources
2. UIElement3D – adds support for input and focus events to 3D elements (finally, no more hand crafting!)
3. Viewport2DVisual3D – enables you to paint 3D surfaces with interactive 2D elements. This was a sorely missed feature from 3.0 that appeared as an open source project later and has now been folded into the actual WPF API as a first class citizen.