I think the spirit of the OPs comment is that it is the style of conversations, atmosphere and culture that each of them foster what makes them somewhat different.
If you want to organize discussion around topics, model it after reddit, where you subscribe to topics.
If you want to organize discussion around people, model it after twitter, where you subscribe to people.
Kbin and lemmy do a good job of modelling things after reddit, where you subscribe to topics. The decentralized nature just adds another layer of community duplication, but that was already a problem with reddit (r/gaming and r/games) and isn't that big a deal since all are subscribable from your preferred instance as long as it's federated with everyone.
The problem with Mastodon though is that it wants to model itself after Twitter where you subscribe to people, but unlike with topics, having duplicate copies of people is a real problem since it makes it hard to trust that you're actually subscribing to the right person and not a spam account. That is an extremely real problem that Mastodon tried to side step by pivoting to following topics, but at it's core the mastodon/twitter UX is not formatted for that, it's formatted for following people in real time and Mastodon seems like it has ignored that and is trying to insist that it's it's own thing that no one actually wants. Organizing discussion based around servers is not a user helpful format, it's exposing unwanted technical implementation details to the user in a way that only a tech nerd could ever love.