Just the specificity of communities you could find.
To name a few I've used frequently the past few months:
r/hardcore
r/doommetal
r/SuccessionTV
r/TheSmashingPumpkins
r/[my college]
r/[the town where I live]
I love lemmy but so far the userbase just isn't big enough to fill a lot of the niches I'm looking for. I'm excited to see the community grow though and I'm hopeful about the future.
I'm going to miss finding old subreddits that are new to me and sorting by top. It seems your can do something similar with Lemmy, but there's just not enough content, yet.