Improv.
Knowing the setting, genre, and tone well enough to rarely be at a loss for what happens next.
Being attentive to the needs, desires, and preferences of the players.
Knowing the rules at least pretty well.
To elaborate: I've three what I would call "great" GMs, and they all shared these traits. They were good at thinking on their feet, knew the rules, the setting and their plot, and were generally good at maintaining the mood or feel that the players expected from the game. They were good at keeping the players more or less focused.
I will note that none of them did voices, had cool props, or engaged in other theatrics much. Looking back, this surprises me, because the games they ran were very atmospheric and often genuinely and appropriately tense.