Teaching new users how to find, subscribe and be able to post to communities that aren't on their own instances is the biggest hurdle in the Fediverse I've come across so far. To find this community for example, all they have to do is put "!reddit@lemmy.world" in the search box while logged in on their instance and they'll find it and be able to subscribe. Also getting the idea across that they don't have to have an account on the instance where their favorite community is in order to subscribe and post there.
Aside from that, maybe a list of communities in the Fediverse that closely match popular subs on Reddit eg: /r/all, /r/worldnews, /r/videogames, etc.
"You’re in a desert walking along in the sand when all of the sudden you look down, and you see a tortoise, it’s crawling toward you. You reach down, you flip the tortoise over on its back. The tortoise lays on its back, its belly baking in the hot sun, beating its legs trying to turn itself over, but it can’t, not without your help. But you’re not helping. Why is that?"