I must admit that I suck at UI design, but I'd rather try and be constructive in the hope that someone listens and comes up with a good idea.
Complaint #1: The experience going back and forth between Aero and Metro is too distracting.
Would a glassy background in the Metro Start menu work better? Not a big fan of fully glassy windows (like the gadget menu in Win7), but the tiles provide a crisp contrast anyway and if the glass is "blurry" enough, it would only let the basic colors of the desktop background shine through. Metro doesn't mandate a solid background, after all. Of course the single Metro apps would still take over the whole screen with their own background.
Complaint #2: The start menu taking up the whole screen is alien to every previous Windows experience,
I realize that unless users are exposed to the Metro start screen, the notion of apps and the store would be defeated, so a direct replacement of the Windows Start Menu is not an option.
A possible alternative, IMHO, would be to have an expanded charms menu while in desktop mode, so that it takes up - say - 1/3 of the screen (in landscape mode, maybe 1/2 in portrait), has a glassy background (to be more similar to Windows start menu) and includes the charms at the bottom, and some of the tiles in the Metro Start screen (the first group, or another specfic group, or even a filtered view with only desktop apps listed in there).
In a sense, it would be a "minified" version of the Metro start screen plus charms, possibly with an easy way to expand it to the full Metro start, for instance the little animated arrow from WP7 (and/or support swiping it out with a finger), and/or an easy keyboard shortcut (press and hold the Win key, maybe?).
Again, UI design is not my thing, so all of this might be an utter disaster. But I'm interested in your comments, to see if there are alternatives that don't call for going back to Win7.