Comment 24 for bug 855532

My suggestion is that Super+H (as in Help) would bring up the keyboard shortcuts help. The screen would keep focus even when Super+H is lifted (I think this satisfies the accessibility requirements). Then the screen would automatically disappear as any key is pressed.

This is implemented in the Scribes text editor (with Alt+H as key) and it is great to use. http://scribes.sourceforge.net/functions.html

PS. I also liked the idea in comment #20 that when Super+W is pressed the windows would have number shortcuts. At the moment Gnome 3 shell is superior to Unity only in the window switching usability.