Andy Roddick is a wonderful and powerful tennis player. He tries hard and does well.
Unfortunately, he lacks some of the qualities that the very best players in the world possess.
First, he is not light on his feet.
Second, he is not as fast as Federer or Nadal.
Third, his game relies primarily on his serve and his powerful forehand. There is little variety or diversity to his game. He is somewhat one dimensional and a bit predictable.