p.p.s. Also, @Gargron changed the Fediverse's overall approach to onboarding by thinking about ordinary users instead of technical hobbyists.
Mastodon wasn't the first Fediverse platform, but it was the first one to aim itself at a general audience. JoinMastodon.org was a breakthrough because it assumed no technical knowedge or desire to self-host.
Another greatest moment was Mastodon's 2018 official animation: https://fedi.video/w/cbQE3NRw76FayQCSdb14TU
Mastodon sold the concept of the Fediverse to many people.