I think that with AO3, it's not about creating a walled garden so much as allowing people to self-moderate by blocking more easily, and indeed for moderators to be able to take action against someone being abusive. That said, I do hope they come out with a per-account switch soon and that people keep allowing guest comments, it's a really nice thing and I know people who don't want to make accounts who don't want to lose touch with authors.