1. Knowledge. I think somebody who is great at ANYTHING has studied it beyond the minimums. An understanding of physiology, what's known about decompression, how the gear we use works, what the things are we are seeing . . . That curiosity and desire to understand is the mark of someone who is great, to me.
2. Skill. I don't think you can term anyone "great" unless their skills are superb. My cave instructors are amazing in the water -- they're like moving holes in the water column. They don't move unless they want to, and always to a purpose. They have incredible buoyancy control and balance. They also have eagle-eyed awareness of everything that is going on around them, no matter how distracted they are. I aspire to those things.
3. Judgment. As Bob says, a great diver knows when to pass on a dive, and when to call one, and how to match his diving abilities to the conditions of the dive. He also knows how to prepare for any given dive and what planning is needed. Prudence is another facet of this quality.
4. Diligence. I think a great diver does what it takes. If something is noted to be suboptimal -- a fraying tie, or a reg that isn't breathing just right -- he takes the time to get it fixed, rather than ignore it or put it off. He keeps his stuff organized and stores it properly. He's meticulous about caring for gear. He takes the time to maintain his own body, too, and tries to make sure it's in the best condition to do the diving he wants to do. (This is probably my weakest area.)
5. Joy. To me, if someone meets all the other criteria but has lost the joy of doing this, they aren't a role model and they aren't great. A great diver can do his most challenging dives with skill and safety, but is also perfectly happy to jump into shallow water and play with friends.
6. Humility. I think a hallmark of truly great people in any discipline is that they remember that they were once beginners, and that they still put their pants on one leg at a time. Arrogance is a marker of insecurity. Generosity, and a willingness to reach out to others, is a mark of greatness.