When doing art reviews at my company our boss really likes it when we close out all the UI so that all we see is the video playback. We have learned to drive using the hotkeys, but it would totally make our lives easier if we could at least turn off all the UI elements with a single hotkey.
Stretch / dream request: A hot key that turns off all UI, stores which UI you had on previously, then when you press the hotkey again all your previous UI comes back.