He knows what he wants

I've been thinking about this for probably three years now and I still can't figure out what it means when someone compliments a director by saying "he knows what he wants." I came across this recently in the extras on Factotum. Matt Dillon said a lot of his ideas got nixed because the director knew what he wanted (and that was a good thing). In Roger Corman's autobiography, there's an excerpt from Shelley Winters saying Corman always knew what he wanted. A recent interview with the Strause brothers on AVP 2 say that working with James Cameron was highly influential since he knows exactly what he wants.

First of all, this probably means different things depending on the person. But I can't help but feeling that it means that the director has something very specific in mind. Isn't that bad? Wouldn't you rather have a director who knew how the scene was supposed to work but was open to various ways of getting there? I still have no idea.