After reading this post of cadhla's, I'm going to post my own list.
People making false assumptions about me. I can't even begin to list the number of false assumptions folks have made about me over the years -- everything from "You wouldn't like such-and-such because you don't like this-other-unrelated-item" to "You don't like 'x' because you like 'y' when the two are mutually exclusive to "You aren't interested in [something I am interested in knowing."
Non-communication. If you want to make me go downright ballistic, refuse to communicate with me and don't tell me why. Every instinct in my body will just make me totally cloying till you stop your assholish behavior.
Prejudice. How the hell can you tell whether someone is a good or bad person solely on their ethnicity or religion or fill-in-the-blank? Having been on the receiving end of this kind of crap, I despise it when I see it. Period.
People who think it's fun to be cruel to animals. I am as guilty as the next person of dispatching bugs that show up in my house, but I don't enjoy it.
People who think their way to live is the only way to live. If there were only one valid lifestyle, we'd all be doing the same things for a living and lots of jobs would be neglected. I have ancestors who came to the U.S.A. to escape religious persecution. I have ancestors on the other side of the family of different religions. I grew up expecting everybody to be different. The "Lawrence Welk Show" was how other people lived.
People who expect folks to read minds. Not everyone grows up with the same body language or clues. If you want someone to do something, be straightforward but tactful. If you want to get off the phone, don't say "I need to go to the bathroom." Say "I need to get off the phone."
People who impose their own morality on you. Just because something is immoral for you doesn't mean it's immoral for me. On the same token, just because something doesn't fit in with my personal morality doesn't mean it's not okay for you. As a subtle example, I don't have any piercings or tattoos (it's a jew thing), yet many of my friends do.
People who look down on me because my interests aren't "grown up" ones. I like comedy, science fiction, radio and TV, making music, and yes, sometimes my humor tends towards the scatological. Deal with it.
People who are critical of my "style." I get soooo fed up with folks who tell me I "overdress" or "should wear xxx" instead of what I'm comfortable wearing or "you don't dress trendy enough" or "you should dress more appropriate for your [oversized] figure." Aaaugh!