People who smack when they chew. Can't they hear this??
People who talk with their mouths full, I mean absolutely stuffed full. What are they, three years old??
Being cut off in traffic. Just die. Die, die, die, DIEEEEEEE!!!
Being interrupted. This is the biggest reason I am quiet in real life. Nobody converses any more, we just talk right over the other person. It's especially annoying when I am asked a question and my answer is interrupted three words in by the person who asked the question. If you want to know, that's cool. If you don't, that's cool too. But since you clearly don't want to know, Don't. Fucking. Ask.
People who can't do basic arithmetic. A guy at my work thinks I'm a genius because I know $2.00 minus $1.67 is thirty three cents. This from a forty year old man. Likewise with people who can't find quarter inch marks on a ruler, or don't know that a 20oz soda is smaller than a two liter. I'm glad I don't work for the IRS.
Basic spelling and grammar errors. I don't expect everyone to know words like anesthesiologist or obfuscation but using it's instead of its is just plain wrong. I have distinct memories of a decade of English classes growing up, and of not being the only one in those classes. Where did all the people who learned English go??
Pedestrians who step off the curb without looking. Statistically I think all these people should have died a long time ago. How is anyone lucky enough to do that repeatedly and live??