Every player will have their specific strengths and weaknesses. I've seen B players who were great shot makers, but ultimately struggled because their patterns and control were lacking. I've seen B players who had great control, but struggled in shotmaking. So it's difficult to pin down one thing that all B players do wrong. Though since the comparison is between B and AA, I would assume that the AA player does most things better than the B player.
Also, I would say that every amateur could use a bit of work on their stroke.