1. How will administrators raise revenue? Will this limit the size, reliability, and sustainability of instances? It seems that the developers are adamantly against putting adds in the timeline. Maybe adds could go somewhere on the side (like on a regular website)? Some admins ask for donations, others are trying have a sign-up fee. Are there other models? Will instances need to be provided by existing institutions (schools, activist groups, professional societies)?
2. How will instances resist coordinated attacks? It seems like federation opens some avenues for attacks (as does open registration)...but maybe it's nothing more than for any web server.
3. In the linked article, the author discusses two extremes: "special interest forums" vs "global real time social network. ". What is the middle? I imagine a structured #network, where each instance is tightly linked to a few others, but most of the network is "over the horizon" and information moves between those instances indirectly. There probably would be some hub instances (perhaps occupied by major media producers). There could also be "protected" instances and "buffer" instances -- where the former emphasizes the complete exclusion of certain people (both internally and in federation), and the later places similar exclusion rules on their own membership, but not on federated instances.

Thoughts? Are there other essays on this?