For me, the most annoying thing about other people'e driving is when they drive really slowly. If they're learning then I can understand, but take this for example.

I was driving down a road with only a few corners, but too many to overtake. The speed limit is 60 mph. The car in front of me was doing 20 (!). The reasons for this became apparent when he turned right and I could see him. He had his right arm hanging out of the window, holding a cigarette, the other hand was on the wheel. Obviously he couldn't go any faster without changing gear, so instead of dropping his cigarette, he decided he'd stay within the limits of second gear. That really annoyed me. I saw the same car again later that day, but this time he was doing about 45 in a 30 limit. Idiot.

I also get annoyed by people getting in the wrong lane at roundabouts and cutting me up. If you want to turn left, don't get in the right-hand lane!

So, what, if anything, annoys you about other people's driving?