Back when I used Windows, the keyboard shortcuts I used the most were probably super+m (minimize everything), +d (show desktop), +r (show run dialog); in browsers, Ctrl+t (new tab), +shift+t (reopen most recently closed tab), +tab (switch between tabs); alt+tab (switch between windows), +space (open window menu, not sure what it's called); shift+F10 (open context menu or, if in the installer process, open command prompt).
In Linux most of these work in most modern desktop environments, but super+r is usually alt+F2. Relatively recently I learned about alt+` which switches between windows of the same type. Don't know if this also works in Windows. Also, I don't know if one would consider it a key shortcut, but alt+click anywhere in a window drags the window so you don't have to move the cursor to the title bar. Middle click usually pastes the last block of text you highlighted. Note that, due to the nature of Linux, none of these are guaranteed to work in every installation.
I have limited experience with OSX, but it seems like many of the shortcuts work if you replace the modifier key with the command key because Apple needs to be special.