I have to say I get annoyed in my car a lot but I think a lot about driving is just lack of courtesy to others. For example there is a T junction near where I live and on an evening the traffic is moving at like 2mph and many cars won't let you out, it drives me nuts, its just rude.
Then slow drivers, they don't give a shit that they are holding everyone up, they will do 40mph on a 60mph B road and fuck anyone who is behind being held up.
Roundabouts where the road to the right of you - cars turning left not indicating but you can't go out because you think they are going straight on.
I used to have to drive on a B road to work, it was about 10 miles on this road and everyone morning there was this Peugeot 405 with two skin heads in, driving at 40mph. They were fucking arseholes. One day I was behind them and there was a straight and he started putting his foot down on this straight so I couldn't overtake. Then a few days later as I went to overtake I saw him give me the middle finger. Why? He was holding me up, what was his beef? Fucking arsehole. If there wasn't two of them I would have stopped and beat the shit out of him and stuck him in my brown suitcase.