Tattoos. It's essentially impossible to avoid them altogether since 90% of models seem to have them, but large numbers of them, or them being huge and very visible, put me off.
Doesn't stop me being very, very into Alessa Savage, though. Her personality and 'presence' do it for me.
Ah, who am I kidding? It's not really a rule, it's a preference. I actually like several girls who have significant tattoos. In all cases it's who they are (or appear to be in the scene) that overrides my otherwise instinctual distaste for tats.