The best characters in novels and movies often leap into your mind from the moment you first see them -- something they say or do sets them apart and captures your interest, and from that time on, you're hooked.
Likewise, when we roleplay, our characters should always have some memorable trait which can hook other people's interest. Too often, I see roleplayers focusing too much on dark secrets that they only reveal to their closest friends, and neglecting those mannerisms which could give everyone they meet an instant and profound impression of their character. Of course, if some people prefer to roleplay this way, that's fine. But to my mind, roleplaying is best when it avoids cliquishness, and the best roleplayers are those whose characters stay with you, even if you never see them again.
When I'm creating my own characters, I think of such memorable traits as "gimmicks," but that word usually has a negative connotation that doesn't apply in this case. For now, the best word I can think of is "quirk." It's an instantly recognizable pattern of behavior or speech that can let others know who your character is right away. Below you will find some of my favorite quirks I have seen people use in their roleplaying in WoW, each of them entertaining and inspiring it its own way.