Sounds stupid but it's paramount to know your camera and know it well.
If you were blindfolded you and holding your camera, would you be able to perform basic functions?
-focus point selection
-change shutter speed
-view blown highlights
-zoom in on image
-change white balace (okay I might not be able to do this one without looking)
Why is any of this important?
The camera is a tool, but it's really more than that... it's an extension of your mind and the thing that executes your vision. It should perform exactly as requested in the swiftest and most efficient manner. None of this is possible however without knowing your camera inside out.
I've had my D3 for almost 2 years now. My how time flies. In that time, I've updated the firmware twice and every time I read the manual and look through the in-camera menus there are less and less functions that I don't know about. With the D3's extensive abilities, I think there might always be something in the menu lists that I won't anything about.
Really though, if you're using your camera regularly, it should feel like an extension of your arm. You should also read the manual occasionally just to refresh your knowledge of how your camera works :)