I really wish the Mastadon README had some more discussion about what exact problem the system is designed to solve. Closest I can find is "A decentralized solution to commercial platforms, it avoids the risks of a single company monopolizing your communication."
Which is great and all, but... what does that mean exactly?
As an end user, I expect that to mean something like "when the org/entity that shuts down the instance your account is on decides to shut it down, your account/data/history is safe/recoverable, and easy to move somewhere else."
But it doesn't seem like that is the case now, and the README gives no indication at all if that is actually the long term goal or not.
Especially since trust is not a static property that an org/server has; it evolves over time. Who I trust today is not likely to be the same as who I trust in 5 or 10 years. And if my social network/identity is impossible to move, then what has all this hard work gained me, at the end of the day?
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!