I seem to have been beaten to this one, but I'll put it first anyway: smoking.
There are plenty of reasons.
Histrionic personality disorder. Look it up. Think about it. No way.
Artificial breasts. One could write essays on why they're a turn-off. I shall refrain.
Eyebrow and lip piercings, followed by nose and tongue piercings, followed by multiple ear piercings and piercings anywhere else except the navel, followed by ear piercings and navel piercings.
Being horribly unfit.
Dishonesty, hypocrisy, paranoia and nagging.
Lager. This is a long way down the list and a very small deal, but anyone inclined to swill down pints of lager is unlikely to be of interest to me in any way.
Make-up OCD. I don't mind make-up and perfume, although both taste terrible and tend to be poisonous, but if you can't face the world without carefully painting your face on your face first or asphyxiating the family or sports team with half a can of Lynx Sexgod Three Day Corpse, you have problems I don't want to handle.
Willful ignorance. You know those people who are proud of how little they know and how tenaciously they cling to their misinformed opinions in the face of any and all evidence, determinedly shunning all attempts to educate them? Well, would you marry one?