Non-Working Features In Longhorn
This is a living “Non-working” features list I will be compiling over time while working in Longohorn. I know the PDC Build is a pre-alpha and in no way expect everything to work. I am just going to catalog issues here I have found as not working and have mostly been conformed via the winfx newsgroups. If you have any issues you've found and would like to add to the list, ping me and I'll post them.
Avalon - Application
PageFunctions OnFinish - Another very cool “soon to be“ feature. At this time the OnFinish method does not appear to close the PageFunction and update the Window or Pane that initially called it. The curious thing is that two samples came in the Longhorn SDK that show the code, but the applications do not work.
Avalon - Visualization
NineGridBrush as a Fill The NineGridBrush only appears to work as a Background item and not as a Fill of a rectangle for example.
Animation while moving window - The animation does keep track of the results of the animation, it just doesn't re-render the visual until you stop dragging the window.
WinFS
Creating Custom Items - Creating a custom or Extended Item Type for storing in a WinFS store is not supported yet. Will be very cool once you can, for now I am using existing Item Types and re-mapping the available properties.

Longhorn Blog by Adam Kinney