I think there is a lot they can do to make moving between metro and the desktop less jarring. They could very easily skin the desktop away from faux 3d glossy semitransparent things to solid colors for window borders and menus with clean edges, icons, and text.

In the developer preview it looked awful to have the black windows button on the aqua 3d suparbar. Why not just make the whole thing black? I think the desktop would look great with solid white / colored windows over a good photo.