Longhorn, no offense here, even though the interface is ugly to some, has fully passed the GDI to the GPU and is already faster, ahem snappier, than 10.3. We need that speed.
I'd also like to see hardware lighting instead of the current alphablending. Therefore real shadow effects. This could be helpful for multi-tasking or Expose if windows moved backward and forward and the shadows changed size.
Perhaps even a new metal theme which reflects the light and has a bump map to make it look like real metal. High end graphic cards only of course. But I think most of us have got that already or will have by then.
Also, Longhorn is using live icon previews of just about every document type. We can beat them there. I'd like icons that actually look like the documents and see them zoom up to full size when double-clicked, not just images like now. MIP showed this is possible.