Someone who enjoys complaining that anyone has deviated from the main topic, even those posters who are old time regulars that choose to inject some humor that most everyone appreciates and responds to.
"Don't be a threadnag,the group always has something funny to add."
"You're such a threadnag, get a sense of humor. Moderator's Pet"
"How long have you been humorless? If you spent less time trying to be a threadnag, you'd see that his comment is relevant, it just went over your head."