Email is my go-to example of all the problems of federation. In theory any person can stand up their own server and interact with everyone else. In practice, there's been so much abuse over the decades that it takes a staggering investment in time/energy/money/expertise to do so. Enough that's it's completely beyond the reach of the vast majority of people.
This problem is so bad that's it's driven a quiet de facto re-centralization of email.
This problem is so bad that's it's driven a quiet de facto re-centralization of email.