people who judge others on their boat.
people who don't check their six for folks behind them
people who don't catch eddies to regroup a dispersed party
people who blatantly disrespect the place (being loud, littering, otherwise not doing-unto-others as is appropriate)
people who bitch about people who want to scout
people who are proud of their ignorance of throwbag use or basic swiftwater rescue
people who refuse to swim or see swimming as a Major Carnage Event
also, guns. fucking guns on the river. 10,000 people go down the MFS yearly and you think you need a grizzly sidearm?
but man, the single most despicable form of boater bad bahavior is the camp-poacher. Son, you better have an injured pregnant lady going into labor after dark if you are moving in on my camp. The first words outta your mouth better be "I'm really sorry, but this is an emergency."
When in doubt, meditate on the golden rule and the fact that most people are on the river to avoid strangers.