Active communities, period.
Reddit started out without subreddits. Eventually grew too big for that so split into topical channels. Then the channel would get too big and a niche would subdivide out. When Lemmy started, or more so when there was the first big exodus from Reddit, all of that was recreated, but without the users. So we have a ton of dead communities that really need to be rolled up and cleaned up.
Love roller coaster covered by the Red Hot Chili Peppers, originally by the Ohio players I think, unsure because there’s been a few covers, but RHCPs is the best. The problem is they did it for the movie Bevis and Butthead do America, and I’m now there are weird license rights so you can’t stream it, but can be found on you tube.