I was on reddit for a very long time. And this is why I started to bemoan when communities would celebrate that they passed some number of subscribers.

/pardon me as I yell at the clouds. Stop now unless you want to read a completely unnecessary rant.

Two of my favorite niche subreddits were absolutely ruined by getting big: mindfulness and foodporn. The former was primarily a discussion about practicing mindfulness, there were even a couple of buddhists who actually deeply studied the tradition that provided very good non-western insight. It was a good place to go get help, albeit occasionally got a spattering of stupid memes, but you could easily get past them. As it grew it turned more and more into just memes, and then was just over-taken by new-age nonsense and pseudointellectual quotes over pictures. Food porn (while never exactly what I wanted) went from often having well-done pictures of good food, to shitty cell-phone shots of oversized hamburgers, half eaten food, and plates of food sitting on counters with all of this shit in the background.

This is just the result of a lack of quality or subject control.

This is just another way of saying "having mods enforcing super strict rules", which then leads to an ossified culture and a bunch of mods high on their power trip. This was also seen on Reddit and StackOverflow.

Unfortunately, the way to avoid "lowest common denominator" issues that you mention is by going to the places where the denominator is relatively small, but big enough to have network effects in its favor. My experience was that all subreddits between 25k to 500k subscribers worked really well without excessive policing. Between 500k and 1M it could still go by, depending on the moderators. After crossing that mark, things started to deteriorate fast.

If we were to scale that to Lemmy, it means that all communities with a subscriber count >= 1% of the total network will fall into "deteriorate fast" territory.