Hmm, those are some good points. I am mainly concerned with the difficulty in moderating the server well (especially as it scales) but your and the other comments have mostly convinced me.

Well, a huge caveat to that is that there are world class Researchers who create constantly adapting intelligent spam filters to keep spam out of inboxes. Maybe the fediverse will have something like that someday! Who knows!

Just to clarify, individual accounts will be in trouble if they subscribe to communities on other servers that break the rules? Or are they blocked by default?