Pretty much the same things that separate a good BJ from a bad one. Too much tongue, too little tongue. Too much teeth, not enough of it. Too much concentration on the head/clit, not enough concentration on it. Too much speed, not enough speed. Etc. Every woman has a different preference for what turns them on, just like with the guys. The guys that are the best at DATY are the ones that know how to adjust their styles for every woman they've been with. Same with the girls who are the best at BJ's.