At first I read "would not like to see replicated" and I almost got upset, haha.
For me:
Fountain pens
Communities for different countries/regions or different language-speakers - on Reddit, I often lurked /r/de, /r/Philippines, etc.
Mechanical keyboards
Woodworking
Oddly satisfying
Star Trek
Battlestations
I definitely second BIFL, Antiwork, and different communities for U.S. states, as I don't often get to connect with other people from Texas on the Fediverse.
Yes to the language-learning one! I really need to have a space to support my goal of learning German. I'm already really missing that resource that I had on Reddit.
r/france was really fun to hang around for language learning reasons! I loved their sense of humour. Their thread about whether the subreddit should blackout (i.e. strike) over what was going on was absolutely hilarious. It went something like:
Mods: Hey r/france we're wondering whether we should strike on...
Redditors: Yes.
I'm starting to quite like this whole fediverse thing. I'm linking one community in one instance in another instance, while accessing all that from a third different instance, and it all works seamlessly