I've heard it's (currently) impossible to post on Mastodon with a Lemmy account due to how both are differently built, unless you're referring to seeing a Lemmy discussion from Mastodon.
There's the features/app part already mentioned, but I also feel like there's few "discoverability". I'm glad Lemmy doesn't has a addicting algoritm, but it's also kind of hard to find new/diverse content that I like, and there's also some issues in the community, but there's unfortunately nothing to do about it.
Why is the F-Droid community on lemmy.ml??? (●´⌓`●)