I have hundreds of driving pet peeves, but at the top of my list would have to be:
1. People who slow down while merging from a freeway on-ramp.
2. People who don't signal their intention to turn.
3. People who wield cell phones while driving.
4. People who ride the centerline with the drivers side tires.
5. People who drive down the center of an intersection approach that is marked with seperate thru and turn lanes.
6. People who actually slow down to 20 mph at school crossings when school isn't even in session.
7. People who drive grossly below the speed limit (driving 25 or 30 in a 45).
8. People who, while turning left, take the turn shallow, nearly swiping the front-left of the car in the oncoming lane they're turning into. Infuriates me when I'm pulling up to a stop and some jackass comes barreling down on me with half his vehicle in my lane simply because the idiot hasn't mastered the art of the left turn.
9. Complete drivers tests (including vision/hearing/alertness testing) for those age 65 or older should be mandatory every two years. It's not ageism, it's common sense -- some people just shouldn't be behind the wheel of a car.
10. People who don't maintain their vehicles, such as being behind some scumbag at a fast food drive-thru and you've got to choke and suffer in their burning oil exhaust for five minutes.