One thing I hate when I'm getting a blowjob is when the girl starts making slurping, slobbering sounds. I'm not sure if I'm the only one who doesn't like that, but I've been with several girls who seemed to make it a point to make as much noise as they could. Yech.
One time I got a terrible blowjob from a girl who kept pulling and twisting my balls while she was sucking. After that, not only could I not finish, but my balls hurt like hell for 3 days.