I don't think most people grok exactly how difficult moderation is, and how that difficulty doesn't just scale, but rather evolves fundamentally as a community grows. Moderating a smaller community is sort of like setting a certain tone within your group of friends and figuring out ways to encourage them to follow suit. Moderating huge communities is more like designing a judicial branch.
For what it's worth, I think HN is one of the best examples of successful moderation. The team balances their roles as curators and moderators well, they seem to work hard at intellectual honesty in their policy design, and they do a fantastic job of combatting marketing schemes and bots. No doubt there are areas where I don't entirely enjoy HN, but by and large it is the most consistently enjoyable community of its size for me.