Besides being fully finished (it's not even on version 1.0 yet)? I'd say topic variety. Currently most of lemmy's content is very techy stuff, which is enough for people like me who are into that, but reddit has active communities for a lot of interests. Lemmy stuff is like, privacy this, linux that, tor this, development that, while reddit is more like, privacy this, conlang that, drawings this, water that, politics that other thing, etc

I think this is the kinda stuff that will come over time as people start using Lemmy, tho. Art nerds, water nerds, bird nerds, etc will come around and create communities for their interests, and people who are interested in those things will come to share their interests too