As a very rough heuristic, it seems that subreddits with subscriber counts below about 10,000 seem to be able to maintain what makes them great (if they can be

As a very rough heuristic, it seems that subreddits with subscriber counts below about 10,000 seem to be able to maintain what makes them great (if they can be said to be great) without moderation. You have a much higher percentage of users who are deeply interested in the raison d'être of the subreddit. Beyond that, you either need moderation or have to make a new sub.