Regardless of how comfortable you might find them, there's never an acceptable time to wear Crocs in public. For that matter, you probably shouldn't even own a pair unless you're already happily married and have given up on attracting the opposite sex. No matter what else you're wearing, if it's complemented by those warlocks of shoes, we're not going to ask you to dinner next weekend.
It's cool to be a fan of something. Hobbies are good, and it shows that you have passions other than yourself. That said, there's a line in the sand that an awful lot of people are dangerously close to. It doesn't matter if it's a band, a movie, a sports team, or pretty much anything else, being too into something that you have literally no control over/impact on is going to make us not want to see you. For that matter, if you so much as mention the words "fan fiction," we're probably going to stop listening immediately.
Poor dental hygiene
Generally speaking, smelling good is just as important as looking good. No one wants to be with someone who smells horrid, so most westernized adults bathe regularly. The area some people tend to forget is the mouth, which can be a bad situation for a number of reasons. Aside from the instant unattractiveness of bad breath, no one looks their best with busted yellow teeth or poor gum:tooth ratio. Take care of your chompers, and that'll be one less thing you'll have to worry about getting you instantly rejected.
Lack of compassion
When you get right down to it, it doesn't matter who you are or what you're looking for, no one wants to be with someone who can't be nice. There's a big difference between playfully teasing someone and just being mean, so it's a fine line to walk, but being a blatant jerk to everyone (or everyone other than us) isn't going to look good on you. It doesn't matter if you're male or female, err on the side of kindness when first getting to know someone, and then let your inner sarcastic asshole out later. No one is going to get the number of a person who's constantly hating on everything.