For me, aside from picking initially between kbin and Lemmy and then picking an instance (and the whole concept of instances), it was not having an algorithmically created feed. It took a bit to wrap my mind around since all of the social media apps and sites I was used to (and still use) provides this.
I was confronted with building my own feed by topic of interest (aka community or magazine) or else.face a firehose of all content from all local or federated instances. I mean, I did it, so it wasn't that big a barrier, but it still required effort and conscious decision making on my part just to set up the thing to be usable. It's probably one of the reasons why I don't use Mastodon that much, because it's easier to join/subscribe to topics in kbin and Lemmy (at least in my experience). Mastodon seems to be for following individuals and organizations, and that's even more work (for me).