As a counter-example I give /r/AskHistorians. I am not a historian and not affiliated with the site. It is very tightly moderated and it works. Questions and comments that are off-topic or low-quality are removed. Answers must meet quality standards. The repondents take more time and effort in the answers than your average university teaching assistant. If you want to tell jokes or puns there are many other subreddits to go to. The result is one of the best forums I've ever seen on the internet. I'm not even a history buff but I admire that subreddit.

I will agree that few subreddits reach this level. The moderators of AskHistorians put in the effort. The responders put in even more.