I think Lemmy and Mbin's interfaces scare some people off. A lot of users don't know that Lemmy has plenty of other third-party interfaces like Photon, Alexandrite, Voyager, and of course, Old.
Mbin lacks these for now, but makes up for it with the rarely-used custom CSS for magazines, just like Old Reddit, and custom JS too! I use mine to create a two-column layout with big thumbnails! Unfortunately, the CSS doesn't federate and is only visible when viewing it on Fedia itself. I'd love to have even a Lemmy-like banner image on other instances.
For the daily word puzzle Wordle, !wordle and /r/Wordle both benefit from having a bot post a discussion thread for each day's puzzle as it's released. I go there every day and post how I did on the day's puzzle. I still post and moderate on the Reddit one every once in a while, but I tell them about the Lemmy one while I'm there. The recently-created !dailygames does this for several daily puzzle games, but I'm not sure if it's a stronger community or just more cluttered.
Of course, we could remind Reddit users that the Fediverse doesn't have ads! That was a huge selling point for me.