Besides the obvious of speed, power and accuracy, I would put patience and mental fortitude on there.
If he is the greatest, then it is based on results, first and foremost. Now if I had to say what quality made him so successful, even relative to the elite field of great heavyweights, I would say it was that he was a complete fighter. He was the only great heavyweight, who was one of the best punchers among them, and one of the beat technicians at the same time.
How he carried his power, and what he was able to do with it. Clearly, to me, that was his outlying trait.
Besides what has already been mentioned, I'd say his great balance. If he missed with a punch, he hardly ever threw himself off balance or fell in on his opponent, so he had to regroup. He seemed to always be perfectly positioned to follow up with the next punch.