BEFORE THE SESSION: Poor TCB, a pattern of NCNS, or known BBFS.
UPON ARRIVAL (no fee is paid): Bait and Switch, Poor Hygiene, evidence of drunkeness or influence of narcotics, a sudden list of "thou shall nots" that was not communicated before the booking--like no kissing, no DATY etc, extreme tardiness without a plausible explanation. . .
I am sure there are more, but these top the list of immediate departure.
On the issue of CBJs, I generally prefer the BBBJ, but it isn't a dealbreaker. I won't repeat however, if the BJ (covered or otherwise) is perfunctory and passionless. Even if you don't like to do that, at least make me think you like it for about 8 or 9 minutes.