I'm using Classic Start Menu on my Surface Pro, but since I use it almost exclusively as a tablet, I still use the Start Screen a lot. For the Windows key, I'd love the option to have it operate as follows:
On Desktop or Desktop Program: Windows Start Screen
In Metro App: Windows Start Screen
On Windows Start Screen: Show Desktop
Essentially, that's default Windows behavior, except the last line, which normally switches back to the last app open (including Desktop if it was last). My way would make it a semi-toggle for Metro and Desktop modes. When in Desktop mode I'm happy touching the on-screen Classic Start Menu button, don't need the (semi-)physical Windows key for that.
Hope that all makes sense, and thanks!