Hey Voyeur, great question.
For me, there're absolute vs. relative dealbreakers—traits that completely disqualify an SP vs. traits that seriously discourage me but still have to be weighed against her pluses.
When I plot my next poon and read reviews, absolute dealbreakers would be credible descriptions of her as a rusher, or grossly obese (I don't mind pleasantly plump), or taking calls during the session, or not allowing DATY, or smelling bad, or refusing to cooperate with reasonable requests such as CBJ or changing position.
There're lots of relative dealbreakers for me (serious disincentives)—including such things as a donation of more than $$.5; difficulty in connecting or setting up a meeting; a reputation for somewhat inconsistent or business-like or mechanical service; complete inability to communicate in a language I can understand; or being beyond an age where women give off strong fertility signals. But, while some guys seem to have a rather narrow focus on looks, I enjoy intimacy with women of a wide diversity of ages and appearances.
But once I've taken the plunge and a pay-up-front session has begun, I can't think of anything except signs of disease (like bad coughing, open sores or unbearable smell) that would count as the kind of dealbreaker to make me stop—short of an SP's pointblank request to do so. No matter how severely an SP disappoints me, my attitude is always to keep my temper and forge ahead toward my orgasm as best as circumstances permit. Of course I much prefer "friendly" sessions but, if necessary, I can even enjoy a "hostile" session, by clicking into a certain primal "I'll show you" kind of mood that taps into my more stubborn, aggressive side.