Lemmy very much tries to be "federated Reddit". It's Reddit as it was in 2010ish, and that's all it tries to be. And that's fine, but it limits the development of what the Fediverse is. You can use a Mastodon account to browse Lemmy, but you can't use a Lemmy account to browse Mastodon (and the devs aren't planning on adding it - I asked).
Kbin, however, looks at things from a different perspective. On Kbin, you have both threads and microblogs. This replicates modern Reddit's ability to post to your own profile, except instead of going to some user subreddit that nobody reads - it's treated like a post on Twitter or Tumblr and shared more widely. You can follow people on Mastodon from Kbin, and vice versa. There are plans in the future to support more things that make the Fediverse great - you can read the roadmap here.
Note Kbin as a project is less than a year old, and this "main" server only came online a month ago. Until very recently it was just ernest talking to himself... this amount of growth wasn't planned for!
Long-term, Kbin will be somewhere that connects the Fediverse platforms - you won't need a Mastodon account and a Pixelfed account and a PeerTube account. I really like that approach. Rather than trying to do one thing to the detriment of everything else, it goes beyond just a Reddit clone and is also its own thing. That's why I joined; it's a completely different approach to how the Fediverse should be interacted with.