TLDR: Mastodon is very white, male, status quo, global north, and designed in a way to limit its potential for use in social movements.

Mastodon feels very different from Twitter, deliberately. It is truly like Live Journal. It is about people who are happy with their social circle and who think of virality and discoverability as a problem. They just want to communicate with the people they know, and limit other things. Ironically, it is more like a private gathering than a public space.