No actor controls a large portion of visible activity.
This is important for instances to be able to defederate from those with bad moderation, harmful values, etc. If a person or group controls a big portion of the content that people see on an instance, then that instance will lose a lot of that content should they defederate. That person or group would essentially be able to do whatever, and instances would find difficulty defederating because they'd lose so much visible activity and thus users.
If a single entity gets enough dominance over activity, they could make defederation from them out of the question for a ton of users. Furthermore, that entity could cripple the fediverse by simply leaving it, taking a bunch of users from other instances with them. This is a big concern many people have with Threads; if 90% of the activity you see on mastodon.social comes from Threads, then Meta would be able to nab a ton of mastodon.social users by leaving the fediverse, facing those users with the choice of either losing a ton of their connections & follows or jumping ship to Threads.
But you don't even need a supermajority of content to cause that much harm. For example, take the threadiverse (Lemmy/Kbin). A large portion of visible activity is controlled by the admins of lemmy.world. Thankfully, they seem to nice people, but if they were to start (for example) being more lax with hate speech, other Lemmy/Kbin instances would either have to deal with it or lose access to a large portion of the activity pool. If any threadiverse instance were to defederate from lemmy.world — even if the lemmy.world admins started acting against the interests of the fediverse and its users — that instance would lose a dangerous number of users.
Users can move between instances without penalty.
One of the main benefits of the fediverse is that you can move to a different instance and still be able to view the same content. If the admins of your instance start making moderation decisions you disagree with or you just decide that you want to be on an instance that you yourself run, you're able to move and still interact with the content pool. Thus, as long as the platform your destination instance uses (e.g., Firefish, Kbin, Mastodon) supports the same type of content as your old one, you should be able to move without any downsides. The more penalty there is for moving, the more people will feel trapped on an instance even if they want to leave.
This is partially a matter of robust systems for moving accounts, but it's also a matter of having good options available. Mastodon has a ton of active, stable instances, so if you ever want to move (e.g., because your instance is or isn't defederating from Threads), you can do so and still be able to use Mastodon. However, the only such instance on Kbin is kbin.social (not counting instances that run Mbin, a fork with different features & development). If you want to move from kbin.social to another Kbin instance, you don't really have a lot of options. And if you're on something that's closed-source, you'll be forced to move to a different platform entirely, which may not be great for the user — an important reason why free and open-source software should be prominent on the fediverse.
Obviously, this is something that might be impossible to achieve. But even if we can't eliminate the strings attached to moving to another instance, we should try to minimize them.
People can create and run their own instances to their liking with minimal effort.
If a user wants to, they should be able to control their interactions on the fediverse through running their own instance, and doing so should require as little effort as is feasible. Many people have already set up single-person instances for the purpose of having more control over their data. If people can't do that, then they're forced to put their account and content under the control of other people. Of course, most people are fine with this provided that they trust their instance admins, but the option to be your own admin should be as available as possible.
This is part of why it's so important to have prominent open-source platforms. If Mastodon weren't open-source, then anyone who likes Mastodon but wants to control their content would be out of luck. If you like the Threads interface but don't want to be on an instance run by Meta, you just don't have that option.