Discovering communities is easily the #1 complaint, I don't think it's a technical issue, it feels mostly a conceptual issue with how everything works. I understand why duplicate communities exist because of how the Fediverse works, but in practice it's pretty annoying to the users. For example I tried to look for an anime community just to see if there's any discussion, but I had no idea where people were.
There's anime@lemmy.ml, this looks like the most popular but it's mostly repost bots. There's anime@lemmy.world and ani_me@lemmy.world, both of which barely have users. There's anime@kbin.social, which has some threads going on but few users.
Because of the amount of duplicates nobody knows where the users actually are. Since everyone's confused, nobody participates because they feel like nobody else is going to see their content. On Reddit you had one definitive subreddit for each topic, on Lemmy it feels like a guessing game at times which one's the right one.
We're settling into communities more as time goes on (like how !moviesandtv@lemmy.film is the definitive movie/tv hub), but I think we've got a ways to go. If Lemmy wants to go more mainstream it needs to tackle this, whether it's through multi-reddit style communities that combines feeds or some way to combine comments on crossposts or maybe some other way.