Granted, he does have a selection of good serves, which he does with almost identical initial movement. While other players have stronger serves, I don't think there are players who can vary the serve quite like JOW. That variation seems to be an important tactic throughout his game, constantly giving the opponent what he doesn't expect.

However, most of the serves in that video were hidden (two exceptions as far as I can tell), and that helps a lot with the surprise factor. Not to detract from JOW's qualities, but it is a lot harder to serve an "ace" when the ball and racket are visible to the opponent at the point of impact.