Also, get rid of ALL the duality. Like the separate task list for Metro apps only: get rid of it. Use a single task bar and task switcher; the desktop task bar seems just fine for that. Also I don't see the point in the charms bar, which is also annoying as it's hidden and pops up mostly when you don't need it. Which, actually, is all of the time: it's really not necessary to have; if anything needs starting, or searching for, use the start screen!
I don't mind if that start screen would have a couple versions to it (or tabs, etc.) such as one for settings / a redesigned control panel.
I don't mind the start screen in itself, it would indeed be nice if it just showed as an overlay with a dimmed/blurred background, I guess similar to what Ubuntu Unity does when you make the dash open full-screen.