it really annoys me when people are rude to service staff.

that being said it really annoys me when service staff are incompetent.
ie. don't know whats in the food, don't know whats on the menu (these rules may of course be bent in 'ethnic' establishments where due to a language barrier you can never quite be sure that they're incompetent).
classic example: "it's okay, there's no vegan in that"

also, when people treat dietery requirements based on allergies/religion/ethics.
ie. potential for a lawsuit.

EDIT: rule of thumb: don't be rude unless somebody's rude to you. but if service staff are rude, it's particularly inappropriate, in which case its a free for all.