A few things that grind my gears:
I work in the library of a college (where the IT dept is housed), students are always trying to open a door that requires ID to enter (all student/college ID's will open the door)....There are a few signs on the door stating that the door requires ID to open, yet they still pull the door...I see a lot of repeat offenders and seniors at the college pull the door.
Another thing, stupid people.
Also, one word/short responses..."How are you?", "Good"....pisses me off