The Poltical, Religious, and Social Issues forum. I liked it, but many found it irrelevant to a tech site like MR and sometimes debate there would get a little out of hand. In the end I think the moderators, who made heroic efforts trying to keep things civil, reached their limit and the forum was shut down. For me that was a pity, for the people posting there were far more intelligent, educated, and interesting than the trolls who lurk in current affairs forums of most social media sites.