Third party client support. Specifically alternate web UI's focused on desktop like Alexandrite because if I'm being honest here, I think the comment nesting in Lemmy's offical web UI lacks enough distinction to be useful on desktop (its clearly optimized for mobile browsers). Following conversations can be frustrating on desktop. Without Alexandrite I'd most likely be a mobile app (Voyager) user only.
Edit: No, third party web front ends for reddit do not work anymore. Remember those pesky API changes that went into effect in July and were the entire reason the majority of us are on lemmy now? Yeah, that didn't just kill off third party mobile apps.
Fewer users.
Once a site hits a critical mass of users the amount of content goes up and quality goes down. Once you reach that point it begins accelerating and turns the whole community to trash.
Language, you can filter content by language you speak
Edit title, If with my broken english and autocorrect, I write down does anybody now about a boardgame for trees ? I can do a ninja edit without deleting the post
Interaction with Mastodon (and the rest of the fedi), seriously, imagine being able to answer to a tweet from reddit, with Lemmy you can answer to a toot
The community is more mature, less stupid pun chains (pretty sure those are mostly bots at this point), and less presence of interest groups (nefarious or not).
Multiple communities for the same topic, but on different Lemmy instances.
This is both good and bad (which c/ do I subscribe to?) but I argue its a pro more than a con.