When I started this thread, I didn't consider the attitude that religion, politics, and sex should not be discussed in polite conversation. (Some versions include money in that list.) They provide an interesting place to start, but I had always seen different reasons to avoid those subjects.
I had always seen religion as taboo because most belief systems define themselves as the only way to reach God. If one truly believes one of those systems, then there is a kind of condemnation of anyone outside that system. I think mature people can learn to be friends in spite of that difference, but most of us don't like having to state that a friend is condemned under our belief system and don't like being seen that way by a friend.
Politics is similar except that the condemnation is in this world instead of in the next world. Someone who wants to take my money (or allow others to take my money) for wrongful purposes is my enemy. Someone who wants to take my freedom without just cause (as I see just cause) is my enemy. I'm less worried about exposing my belief in the sense of letting someone know what I believe than I am about having to define someone as my enemy.
I had always seen sex as a forbidden subject because our society has traditionally been much more modest. The sexual relationship between a man and his wife was only for the two of them to understand. Discussing the topic was considered immodest and an invasion.
I'd like to hear others' opinions of what makes a forbidden topic into a forbidden topic.
I notice sometimes a forbidden topic can be like a superstition. For example I used to work at this big place where people could come and go. This one woman was just pure evil and was hated. About 4 times I heard someone mention her and begin to talk about her and another person (a different person each time) would say something like, “Hush! Don't mention her. Don't conjure her up! Don't talk her up!” Meaning sheer talking about her may uncannily bring her around.
Also I have seen this with employment anxiety and I must say I take it seriously myself. At work or away from work it can be very uncomfortable for people if you say that you have a secure job and will be there forever or your company is rock solid. That is like asking for trouble. I am superstitious enough that I'd never say that.
Another difficult subject can be health and I learned this as a tot from my older relatives and fast-forward because I practice it now, too. That is if you've been ailing or have something wrong and somebody asks how you are doing it is almost like if you say, “Oh, I'm all better. I'm much better. It doesn't hurt anymore. etc., etc.” - your problem will come back within hours!
I don't know what to say about these ideas. I've experienced them so I know people, at least some people, heed them. It is almost like words have power.