What makes a good AD?

Every AD I've worked with has fallen into one of two categories. They were either hyper competent, driven and organized people, or complete control freak megalomaniacs. I just got off a set today where the AD spent a few minutes arguing with the DP on what F-Stop to put the camera at for some reason or another. This struck me as way out of line but I'm curious to see what other people think.

What are some of the qualities that make an excellent AD and what are some telltale signs that the AD is insane?