^ She'd scare me, if I were on the other side of the net.....
Serena has power advantage. This translates to
1) Faster service getting free points or setting up the point for a forehand put away
2) Faster backhand / forehand giving opponents less time to get to the ball leading to either an outright winning point or make it easier to put away the next ball.
Fortunately, this is offset by her inconsistency to let the other girls back into the match. However, when she is in the zone then her tremendous power advantage (hear her roar!) will net her easy wins most of the time.
Na Li who took Serena to 2 tie breaks said that the difference was Serena's service.