being a GM is not all it cracked up to be =\...........
its nice feeling like a king and sitting on your throne...but once that celebration is over.....the king needs to listen to his villager's complaints....and work out a solution...........every....single.....day..........
But if i had to guess....i'd say a nice well known knowledge of the game, data, sprite, game play, and functions...also know more than the players about the game, and when faced with a problem you can calmly and quickly solve it...