Too many to list, but i can sum up: people that aren't purposeful and thoughtful. Just quit screwing around, pay attention, and think about how your decisions affect others.
My biggest peeve: people crossing the centerline. Unless it's dotted and you are passing, this is how people die.
I think we need to make it much harder to get a license in this country. That, with much stiffer, and better enforced penalties, would reduce society's cost. Less road maintenance, fewer collisions, less DOT help for broken down cars, etc.
Driving is not a right, it's a privilege.