1. Idiots with mullets that shout "Run, Faarrrr-ust, Run!" or ask "Hey buddy, whar's the far?" - I live in a small town in rural Alabama and this is way too common.
2. Smoking course workers - I am very tolerant of other runners, especially at large, crowded races, as I remember it wasn't very long ago that I was the newbie out there unknowingly breaking all the rules of running etiquette. However, I absolutely hate it when you are approaching an intersection and have to run through a cloud of smoke because the policeman or city worker that is directing traffic felt the need to light one up.
3. The competitive runners that finish the race and then turn and run back up the course in reverse to let everyone else know how far ahead of you they finished and that they are now 2 miles into their training run because the race wasn't long/intense enough for them. I may just be ignorant and this is something that elite/competitive runners are supposed to do or something, but it drives me nuts. It probably bothers me so much because I am just jealous. ;-p
4. Dead things in the road.