What's the difference between the "Win-D" and "Win-M" shortcuts?
"Minimize All" - WinM, that is - sends a minimize command to all windows. However, some windows don't support the minimize command, and will thus still be visible.
In those cases, WinD, "Show Desktop", will get you to the desktop. Note that the windows that don't support minimizing are still there, unminimized - they just become invisible.