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.
askhistorians is one of the only counter examples to my point, specifically because moderators have domain knowledge. they are nowhere near as easily replaceable. iama mods are not expert interviewers, they are trash collectors and coordinators.
something else you will notice about askhistorians is that it doesnt have the usual guard. you dont see greatyellowshark, britishenglishpolice, klyde, krispykrackers, manwithoutmodem, maxwellhill, agentlame. the same reoccurring 100 people have taken control of a large portion of reddit (measured by subscriber)
I think part of the reason askhistorians works so well is because it exists outside the influence of that cabal. They come to their own decisions instead of blanketing policies over hundreds of subreddits at once.
I will agree that few subreddits reach this level. The moderators of AskHistorians put in the effort. The responders put in even more.