In practice it's still a desert.
The problem isn't how to sign up, it's that with the exception of a few active communities, there's not much going on once you do.
It's not seeming like much of a desert, and is just starting to benefit from network effects as some of the more known members of the community announce their moves.
https://mathstodon.xyz/invite/5nKykTB6
If it doesn't work then check back in a day or two, many servers are struggling at the moment but the mathstodon admins are scaling up now.
Or you can join any other instance since any account can follow any other account regardless of instance. The main advantage of mathstodon is LaTeX support.
currently not taking signups:
I’m on fosstodon which has this as the local timeline.
https://fosstodon.org/web/public/local
On many servers I can do the obvious substitution to see that server’s local timeline. I don’t think I’ve seen any servers that link that from their top level page.
This presents me with a log in form.
> Due to the massive influx of users from Twitter, we have had to temporarily close registrations. Please check back later if you wish to join Fosstodon.
Under the covers this sets up the federated follow relationship, but the UX feels the same as following someone on twitter.
There is one slightly more complicated scenario; if for some reason you're exploring a different server and find someone there you want to follow. In that case you still click the follow button but have to provide _your_ username@server string which triggers the necessary back-and-forth to get the follow relationship set up.
How would you do that on Twitter? Follow the hashtags. Why do you insist on limiting it to a server? That's not really how it is supposed to work anyway, it's not those things you mentioned.
Otherwise for someone who posts frequently and has a good following, then it's probably worth it to start posting on Twitter and Mastodon at the same time, that's the only way it happens. Big movers and shakers are either going to stay on Twitter or drag everyone else with them, basically unlikely to happen.
That's not true anymore: I am currently following over 100 academics and scientists who decided to migrate over the past week, they are very active and as a result my timeline is much more interesting and engaging than Twitter.
I have never seen such a big exodus in my 25+ years online, this is historic.
Instance admins can certainly be more decisive and just ban anyone they want without causing a media storm. Not sure if that's what people meant when they asked for a Twitter alternative.