Reputation, nb of members, nb of members in your contact list, activity, etc.
Discussion
I still think private relays would be a much better solution for this kind of thing, and more decentralized. You would just have a relay for your community, like a discord. Then the moderation is simply whatever techniques they want to employ of the relay side (paid, friend-of-friend-invite, open, etc).
We need better tools on the client side for filtering views and talking to private relay communites, and not treating them like whatever is in your current pool.
How are topical relays not the same as mastodon and Lemmy? It's a huge entry barrier and the if notes only live there, you can actually get censored.
Not to mention the UX of adding topical relays on your clients and managing different relay feeds sounds like a nightmare compared to nostr communities.