So I was just giving Mastodon a look and was going to open an account... First message: "mastodon.social is full! please select a different server". Ok? so I go to find a different server with a list of topics on the left, "general" selected and a bunch of servers, that don't explain to me why I should join and often are described in languages I don't understand. At the top of the page there's a message: "New here? Check out the help section below.". Help section is not linked, text "help" not found anywhere else on the page... you've to scroll through all the servers to find it.
This app will never be able to get a remotely significant share of Twitter users like this. Most people don't care about it being decentralized and the servers with different moderation policies and how you can read feed between the servers or on the same server... they want to signup quickly, follow accounts they are interested in and see a feed.
Some constructive feedback: the decentralization can be mostly hidden. For example (if it really has to be) users could be asked about their moderation preferences, language, things like that, and be auto-suggested a server (with a button somewhere to change it if it happens to be a "nerd" user), no need to read help and explanations: that can be explored *after* users have signed up and found and followed accounts and are using the app.