In addition to the backend, I'm not sold on the terminology used in the front end either, though visually it does look good.
Why call communities Magazines? Why am I starting a microblog when I press new Post? Why is upvote called Favorite and what does Reduce mean? And what the hell is Boost and how is it different from Favorite?
Still, the number one issue at the moment for sure is the slow federation and syncing with Lemmy. Communicating across different Lemmy instances is no problem, but Kbin<->Lemmy seems incredibly slow, with threads from Lemmy often lagging many hours behind when viewed from Kbin which makes it impossible to participate in conversations.