Any site where users can create and moderate their own boards (reddit, 8ch/kun, etc). I don't think people realize just how much of a misfeature it is. You almost always end up with 15 boards dedicated to the same subject, each with a different take on it.
You're not going to have a board for politics, you're going to have a board for liberals, a board for conservatives, a board for communists, a board for anarchists, and a zillion other boards where you can go to avoid interacting with people you disagree with.