The concern I always have is that the fediverse is amazing and wonderful, and I don't want anyone coming in and breaking it.
The mastodon side usually has a bit of a culture shock because mastodon isn't Twitter, just like Lemmy isn't reddit. People keep acting like it's Twitter and they have a reality check fast.
I think you think you could probably write an entire textbook on internet culture and how the two end up different in practice.
A few key differences:
No algorithm on mastodon means you see what's posted or reposted by someone you follow, not what the algorithm thinks will piss you off
Different instances and defederation mean that certain communities don't cross paths. There's an iron curtain between certain sets of instances. Those blocks are all one-way, because for the most part dark fedi doesn't block. There's also all sorts of intra-instance politics unique to the fediverse and not related to Twitter in the least. All kinds of fedi deep lore because people have been living together in the space for years independently of big tech.
Theres specific cultures that are intentionally unintuitive to outsiders. For example, lots of users on the so-called dark fedi are intentionally crass. People will look and see they're using slurs and Nazi imagery, and while there's no doubt actual bad people out there, most of it is just meant to scare off the establishment types which most of them deeply mistrust which is why they came to a decentralized self-hosted platform in the first place. There won't be anyone getting banned on those dark fedi instances because their admins are in on the joke and don't want establishment types hanging around either. That's just one fediverse subculture of many, and they were here long before Elon Musk owned Twitter and long before spez said a word about any APIs, so their way if being has been. Independently evolving for a long time.
There is no big brother controlling everything that anyone can appeal to. Mastodon.social is by far the largest instance and for much of the fediverse it could defed from them and they'd barely notice. New Twitter users get this ideological vision that everyone will defederate from the bad people, but being decentralized and run by all kinds of different people so nobody agrees who the bad people are or whether they should be defederated from. Ultimately your instance chooses its own path and that's it. That has a big effect since what is possible determines what will be done.
There's no fediverse-wide search. There are searches on different platforms, but unless your instance is subbed to everyone everywhere, you can't search for everything. You can only search for what your instance knows which is almost always a tiny fraction of the fediverse.
That's a few examples of the big cultural differences. There's a lot more and authors cleverer than I have written about it in the past.
Mastodon is (IMHO) like twitter was when it started. A SOCIAL network, where people talk to each other, respectfully. There's no follower fishing, as there is no way to get famous. All you can get there is respect.