I consider a topic taboo if it meets the definition in the first paragraph of the first post in here. Most of these tend to pertain to sex, violence, politics, or behavior that defies social norms. Though some things are more taboo than others. I doubt many people would care if I played an amazon tribesman who walked around in the nude. But if I made a rapist? Yikes. Just saying rape is enough to trigger some people.

The trick to dealing with taboo topics is to know your audience, and also to have a bit of tact and creativity. It might surprise you to hear that as taboo as sexual assault is, there's actually a lot of cartoons that depict it. I know everyone's seen one of the hundreds of instances where a cave man clubs a woman and then drags her away to be his bride. We all know what's going to happen, but it hasn't stopped most of us from laughing. You can get "dirtier" and more serious than that if your RP group/forum is more accepting of that sort of thing and you know what you're doing.

As with all things, it really depends on how it's handled and what your RP group is composed of.