I think it is the ability to use the best strength you have and then improve as well as beating the best fighters there are. A man in my mind cannot be too high on the ATG list if he does not beat other ATG fighters. Ali beat several and so did Leonard.
Heart, determination, motivation to be the best. Does that mean you have to beat every one you come across? Nope. But taking risks, taking chances, making the most of what you have got. Giving your all in every bout. Longevity. Producing a great resume, win or lose. Overcoming adversity. Beating fellow/future greats.
Ring magazine put it perfectly, To become a great fighter you must meet & defeat other great fighters in their prime.
maybe Floyd can someday try and fight ATG fighters who are still great. That is what he lacks. The boxing public doesn't care if a fighter fights greats and loses trying. All the greats lost and some lost pretty onesided. But the boxing public will not look at Floyd too favorably in regards to boxing history if he does not try to fight the best fighters like Pacman right now. Who cares if he loses. Someone has to lose. Just fight the best.