The Ideal GM: System Mastery

It's a bit of a loaded term, but one of the qualities of the ideal GM is complete knowledge of the rules of whatever game she happens to be running. Obviously, the best we, as mere mortals can hope for is an extensive knowledge of the rules, but it's still worthwhile for a number of reasons. Knowledge of the rules helps us design adventures and settings which are consistent with the ruleset, and the more we know, the faster and more efficiently we can do it. This consistency can be one of the cornerstones of our credibility, which allows us to act in secret without the worry that we're not playing the same game as everyone else in the group. However, what's interesting about the ideal GM on this subject is that she goes above and beyond this, using her system mastery in other ways in order to make the game more fun.