Features and speed are cool for keeping the community unified and organized, but I don't think I would ever join a generic forum just because the site looks neat. The features should serve the niche the forum is built around.
Tildes has a fairly nice mechanism of notifying you about comments to your posts that I think draws people in to keep participating. (Note: I still have Tildes invites)
Transparency and auditability.
* mastodon which is a federated "twitter" * pixelfed which is a federated "flickr" * Kbin and Lemmy are a federated "reddit"
Lately I've taken some interest in "Web3" in the sense that I think the Fedi is missing single sign on, but Web3 is problematic in many ways, not least the financialization/ownership thing seems to be about as offputting as having a friend join Amway, and that even with a good solution to signing your signing key people will still struggle to manage keys. Client-side SSL certificates have been around since the server-side ones but when have you seen one used?