A lot of questions, I can only answer some
I think we will try to include most sites, as long as they look like reasonably useful communities for the general public. The goal is to eventually allow people to search for communities based on interests they have, that can be through keywords, semantic search, tags or possibly something else.
We will exclude communities with content we find objectionable. Anything illegal or discriminatory and the sort, for sure.
I personally don’t think we should add nofollow to the links.
Interesting question, I hadn’t thought about this, I must admit.
Thanks for bringing up this experience and example. We don’t have any plans to do this in v1, but this certainly sounds like a great v2 or v3 feature. Thinking out loud here, we could reasonably quickly get to a place where we have a “hot topic” list populated by the participating forums in the network. We’re far from that in this first iteration, but I think it’s a great long-term goal.
One additional piece of information that is somewhat relevant to your questions is that we plan to integrate the Discover page in our DiscourseHub app on iOS and Android. It should make the app more useful and interesting to a wider audience. New users can browse the possible communities as soon as they install the app. And existing can expand their Discourse forum exposure, if they’re already using the app for Community A, they may find communities B and C interesting too.