The sign up process is kind of user hostile (or at least was a couple of months ago, they improved it a lot). People don’t really want to have to figure out which server to sign up to if they barely know what a server is.
They’re probably not talking about the actual conversations on Mastodon (although there’s definitely some condescension there, like if you dare to post an image without alt text they’ll come out).
I feel that about the Fediverse too, despite being highly technical. Picking a server involves trust (e.g. that they won’t go offline and take your account down with them), but you’re just exposed to a list of servers with no idea who runs them. Plus, the server name shows up in your handle, so it affects one’s public persona and people care about that.
I’m on Lemmy through lemmy.ca which feels like an authoritative “Lemmy for Canada”, but… it’s just some random individual person who snagged the domain name. They seem great but I have no assurance that something weird won’t happen with it later.
