I'll tell you what really grinds my gears.... bad drivers, especially people that almost slow down to the point of stopping when making a turn. Slow down, sure, but not to the point where cars at the light behind you have now caught up and I can't turn because of your slow self.
That's why I enjoy my morning job. Nobody on the road for the majority of the time.
I'm surprisingly Zen with the driving I do. The only thing that gets me cranky is the rolling roadblocks I get behind on the four lane on my Bedford run, with someone going 67 in one lane alongside someone going 66. Fvcking cruise control.