David Harris is an absolutely fantastic run defender. He has shown a great ability to fight off blocks and work in traffic. These were areas that gave the smaller Vilma problems. It is difficult to appreciate how good Harris is without watching him on a weekly basis. He doesn't do anything to make one's eye pop out. He is just consistently good at reading backs and getting to them (AFC Championship Game notwithstanding).
His impact cannot be felt with just numbers. I think back to the game against Minnesota this season. Harris was one of the biggest reasons the Jets won. Adrian Peterson might have run for over 200 yards had Harris not stepped in and made a number of key tackles to limit gains.
Some numbers do tell the story, though. David has led a top ten run defense in tackles for two straight seasons.