What will make something like Mastodon actually succeed and not go the way of app.net or others?

I also recently tried switching from Instagram to iCloud Photo Sharing and while it seems like be a viable alternative, it lacks polish, maximum ease of use, frequent updates / evolving features, and cool factor. Many of those problems seem to apply to Mastodon vs Twitter as well.

Mastodon

Server run by the main developers of the project 🐘 It is not focused on any particular niche interest - everyone is welcome as long as you follow our code of conduct!