The first hurdle for a new user is deciding on what server to join, without knowing what that means or how to decide between them.
Second, they have to figure out who to follow and how to curate their feed to the point where it's rewarding, before losing interest.
Third, Mastodon doesn't have communities / groups, which is what a lot of users use especially Facebook for.
Fourth, the tools for discoverability are weak. Sure, there's /explore and hashtags, but not much else.