Politics and Religion
Religion and Politics. Religion because I am a Catholic in an overwhelmingly Protestant area. Politics because I found the last two presidential candidates equally disgusting.
Posted on FB, the opinion that there isn't necessarily a topic he avoids, but there are people with whom he would avoid certain topics. So, different conversational topics with friends vs. coworkers vs. family. In the online world, this worked out okay with Google+ since it allowed you to create 'circles' of contacts to have discussions with. But something like Facebook is too broad since all your contacts will see your post.
A note on religious discussions: it can be tricky to navigate, and really depends on the nature of the conversation. If the people discussing it are genuinely curious about the other person's perspective and will accept that they may have different beliefs, you can have really good conversations where you learn about people. But if the tone is having to defend your beliefs, it likely won't be a good experience.