The main technical feature on reddit absent on lemmy that I can think of is live chat, but if Lemmy does that at all, it should probably be done somewhat differently.
Mostly it is about the user population. Maybe Reddit's is getting worse now, but for a while it was at least a little bit representative of the real world, while Lemmy has always been niche. That's not just a matter of numbers, either.
Fuck that live chat shit... It didn't belong on Reddit even and just felt forced or shoehorned in because the idiots in charge eventually thought that's what "social media" should have.