From what I remember, MacClellan's biggest problem was that he refused to budge until he had every last soldier, bullet, and bootstrap ready to go... and when he did budge, he was hesitant and tended to vastly overestimate the opposition. Did he ever have the chance to fight a purely defensive battle? He was pretty good at building defensive lines... if Lee decided he had to move to the north and for some reason attacked into Mac's defensive lines, the Union might have kicked butt and taken names. Still, Lee would be more likely to maneuver around the lines, I suppose....