Maybe I'm naive but I don't like broad defederation from a platform. That's what will ruin the dedicerse when the money and large companies creep in. Imagine Sony vs Microsoft video game wars with exclusives. It's a system designed for decentralized enjoyment of all. Banning/blocking instances from other instances by default is problematic to the health of the basic idea of federation.
Leave the decision in the user's hands, IMO.
Team Mastodon over here. Best part is I don't have to explain why a federated social media is the way to go on Kbin.