The question becomes how many relays specializing in a particular type of content will share notes.

Right now, websites don't really share content well with each other. With relays, that can be changed, as sharing is implied in the data model.

Could a single relay choose not to share, and would users/clients validate that approach by subscribing to it?

Are we already seeing centralization in the services that store images, videos, and other data content?

Things to keep in mind, questions to answer continually as we move forward in building out thia new communication galaxy.