Intro threads. They just kinda irritate me. Either they're too cookie cutter and do nothing to identify or differentiate a person from the dozen other people who say the same exact thing, or they're too "original". For instance, phrases like "Introducing me" or "I'm here!" as if we've been waiting for them and they're a celebrity, are a dime a dozen. Along with "fresh meat" and "come get the newb". I've seen the phrase "new here but not new to role-playing" here and everywhere else I've ever RPed so much, that I've started to find relief when the person says they haven't role-played before.
Or the overly creative types who try to act out in character personas while NOT in a role-play. It's very awkward to read, almost like someone introducing themselves to people at a party and talking with an obviously faked British accent... but they keep it up all night as if it were real, despite admitting that they're not originally from Europe or anything.