I haven't used these myself, but I read that discussion on betting sites is less emotional and troll-y. Presumably because people are more focused on being actually correct, rather than on ideology.
Sounds like it might not even be intentional, but sometimes it's just really wanting something to happen, or betting on something because you really expect it to.
Very interesting observation, my own experience with GJP, metaculous, and other betting fora agree with this.
I wonder if an HN-like forum with prediction at its core would have legs? I always found the best comments in prediction tournaments at least as interesting as the actual estimates, but none of the platforms really support back-and-forth conversation.
Example: https://www.predictit.org/