After reading a post Selgin made I thought what'd be AWESOME is if the main window, sans toolboxes and stuff, could be CLONED and then dragged over to the second monitor to act as a full-size preview. And the window should be active too, so any menu item or tool you choose, you should still be able to manipulate objects in the cloned window. Much like the dual and quad-views that already exist, but be allowed to break those windows away independently and as I say...be awesome.
I think it'd be such a neat feature I even drew a picture to demonstrate:
