Activity: A dead or inactive community is obviously no fun to participate in
Moderation: If you want to see how a complete lack of moderation can play out go check out some usenet groups. It can quickly turn into a cesspool. I do have issues with power mods who get a swelled head and start banning people because they disagree about something or other petty stuff like that. But I think making sure the community is friendly and having some enforced ground rules is necessary.
Experience: The interface itself matters a lot. It's why some people love reddit and others love twitter and others love Instagram, etc. I think actually making the community easy to engage with and digest the content and conversations is vital. Tangential but I remember looking for a community to talk about a certain college football team, and it had 3 sites/message boards with active communities. One of those sites had such an awful user interface that while it had a seemingly active and nice community, I never wanted to go back.
If those three things are in good shape I generally have a good time. When one of those aspects become problematic it can ruin the experience.
I think whatever rules are made won't matter if there can be another server that breaks them and moderation is not something you can subscribe to. The simple reason: scaling. Reddit used to be OK, until facebook did something stupid and suddenly facebook users swarmed the site.
Mastodon is currently going through this: it was pretty tame before Musk + Twitter happened. Twitter is the biggest trashcan I've ever seen and now the trash is migrating. Currently they are mostly sane people migrating, but the twitter mannerisms have already settled in: harrassing mastodon developers, "twitter had this feature, why doesn't mastodon have it", "I'll block anybody with X in their profile", "if you're with X then you aren't with Y --> block", etc.
The same happened to a few forums I was on - eternal september arrived and the quality dropped.
You raise a good point. I'm not sure how moderation is going to play out on Lemmy as it scales up in population and users from reddit and other sites who come in with their own mannerisms and expectations. I think the ideal that's hoped for is that if a certain instance is causing issues with it's users being toxic on this instance we can unlink from them. But say everyone ends up clustering on only one or two instances and one of those communities becomes toxic. I can see how unlinking from them can turn into a really controversial move down the line, especially if a lot of the communities we are subscribed to and like happen to be hosted on that same toxic instance.
Having seen a few subreddits get taken over by tyrannical mods, I’d like to see some kind of active oversight, or meta moderation. Not sure how that could be implemented. Maybe a voting consensus built into the sub / magazine / community.
Activity and the user experience are both important. Variety too. Special Interest Groups are important.
Region focused communities are another area that deserve attention, though they could be served by geographically based instances.