I don't think there are many things MUSH-related that vex me as much as witnessing one player tell another how to play their character. Usually this is derived from some personal inclination that dictates that the person giving the lecturing feels that if that were THEIR character, THEY'D play it like... What vexes me more is when people take IC matters personally OOCly. For example, when Character A insults Character B, and then Player B decides to take it as a personal slight. That's where the line between that which is In Character, and that which is Out Of Character, must be drawn.