There are several aspects to consider when making the distinction between a good and great climber. Primarily, a great climber is well versed and skilled in all aspects of tree work: climbing, cabling, removals, snip and clip. He or she can ascend a tree fluidly, manipulating a variety of techniques: footlocking, ascenders, hand over hand, body thrust and spikes. A good climber relies on technological means, whereas a great climber can make do with the bare essentials; a rope, saddle and saw. A great climber is versatile and can hand snip a boxwood or remove a 110' Tulip with equal ease.
Secondly, overall work practices determine the quality of a climber. Being well rounded, having the talents and skills to pay the bills will only get you so far. What you do while you're in the tree is just as, if not more important than how quickly you ascend or how knowledgable you are. You can whip up a tree and be able to dignose a particular fungus, but if your flush cuts look like crud and you just mangled a shed, no amount of knowledge or skill will ever make up for your work habits.
Third, attitude is also an integral factor. Will you take a back seat and pick up a rake in order to train someone? Do you love what you do? Do you take pride in your work? Do you suck it up and do the job, even though the situation is less than ideal - (ie. where's the crane?, who the hell bid this job? look at the size of those hornets, what kind of ivy is that?, Is that dog poop?, Como se dice "let it run", I drank what last night?).
In summation a great climber is skilled, talented, well rounded, has good work practices and a positive attitude. He is always prepared, both physically and mentally. He's always equipped with the proper tools. He always has an efficient course of action, but that usually is altered upon entering the tree - pregame srategy and half time adjustments. A great climber is pragmatic and can flex to work in all situations. A great climber is efficient and conscious of safe work procedures. A great climber makes work look effortless. And a great climber never poops in the chipbox.