Thus far it's been security-via-obscurity plus aggressive moderation.
How much time do you spend on moderation? Seems like it would be a lot of work. Problems with spam?
That's what interns and crowdsourcing is for. We've got a nifty "bury" feature that lets you hide stories you don't like just like on a lot of these sites.
Cool - Every major city should have one of these. Nice Job Man!