I feel like the niche communities will come with time, so I'm not super worried outside of what happens to one specific writing community's audience, which matters a lot if you're a writer trying to build an audience, particularly if you don't want to wait a few years for the community you're been writing a book for to grow again.
What I'm really missing is the ability to browse /r/all, which will undoubtedly be harder with Lemmy/Kbin. Having something that can aggregate those well is going to be super important for federated communities to snowball together.
Sayonara, Zetsubou-Sensei has some bangers.