I agree with some of the complaints mentioned here. The biggest hurdle to joining Mastodon is deciding which server you actually want to join, which has a big impact on your experience and isn't (and probably can't be) clear to a new user. I'm not sure what they can really do for a better onboarding experience. Perhaps one of those "pick which topic most interests you" wizards, which then will give a user a list of servers that have identified themselves as catering towards those types of users. Even then, I don't think this will really help things, and will probably just drive traffic to the biggest servers.
As for some of his other issues, I've been pleased to see that de-centralization hasn't happened largely, as there's been a large number of new servers popping up. I thought as well that Twitter exiles would likely congregate to a few large servers, but I've seen a pretty diverse listing of servers in my own feeds.
> Your handle on Mastodon is more complicated than on Twitter, because you need to name your server also.
I actually like this, it gives a sense of what types of topics the user is interested in, and what their profile is likely to focus on.