Let's get this one out of the way. People who are rude to the crew and staff. I have to go in to "Serenity Now" sometimes to keep from going off on them.
I remember reading a reply from a member here who witnessed such rude behavior from one of his fellow passenger towards a crew member. I also remember he came up with a good line he told that passenger (words to the effect): "I thought slavery ended a long time ago."