The thing that drives me nuts the most is slow people who stay in the left lane in the passing lanes that are sometimes put in on 2 lane highways (ie, 2 lanes going in the same direction for a time). Some states (Minnesota for sure) simply make a solid line to shunt drivers over to the right lane and it makes a huge difference, but around here (Wisconsin) they don't do that and inevitably I'll end up stuck driving 45 on a behind an old camper on a 2 lane highway because they refuse to keep right.