His ability to block punches with his face.

Everything Hatton does Mayweather has seen and/or does better. Footwork, speed, defense, strategy, the ability to adjust, the ability to break fighters down with a body attack- all favor PBF. Hatton MIGHT be stronger, but that's in question considering Floyd is content to give one at a time to keep from taking one. But he is the naturally bigger fighter and it stands to reason that if an opponent leaves himself wide open he may actually be able to hit him harder.

And many people consider it a slaughter in favor of Floyd because there is such a disparity in their pedigrees.