I often ask people if they're gay/straight/bi/whatever. I know I shouldn't do that, because your sexuality is nobody's business but your own. But that's the one thing I'm nosy about.
People ask me rude questions all the time, mostly about my size. They usually don't realize they're being rude, even when they're just joking, but I have a thick skin and don't get offended unless they say things that are downright mean. It's ok to ask why I'm so short or what size shoe I wear, but don't imply that I'm defective, weak, or incapable of functioning in certain ways. I have no known medical reason for my height, I'm a lot stronger than I look, and I can take care of myself, thankyouverymuch.
I think the rudest thing anyone ever asked me was back in high school gym class. They did an episode on 20/20 about bullying in my school a year or two before this happened, and the girl was one of the bullies who harassed, threatened, and assaulted two girls. The school only suspended the bullies and basically urged the victims to switch schools. I also got my two seconds of fame, since they filmed my bus unloading on the first day of 9th grade. Anyway, this girl was in my gym class and automatically assumed I would be an easy target. So she'd come up to me and randomly say mean things, and I'd either ignore her or say really weird things to throw her off-guard. (I never take bullying personally, because I know bullies are weak inside, and they take their own insecurities out on other people to make themselves look/feel better.) This strategy worked pretty well, because eventually she left me alone and actually avoided me. I overheard her telling some other girl that I "scared" her. Muhahahahaaaa, mission accomplished. Anyway, one day, she randomly asked me, "are you on any medication?" When I said no, she said, "well, you should be."
I also think it's really rude when people ask me really personal questions that are none of their business (especially if it's someone I barely know, or family members). Unless we're good friends, don't ask me serious personal questions about religion, sex, gross bodily functions, and other things you're not supposed to bring up at the dinner table.