Reddit could operate without subreddit moderators. The main reason mods exist is to remove abusive users and bots, both of witch could be handled by the vote system.
I don't think Reddit could operate without moderators. On a technical level, sure it's possible, but as a business they would not be able to operate. If the content didn't have a reputation of being vibrant, interesting, and reliable, no one would use the site and they'd have no income. Reddit's business is only possible with moderators there to cultivate their communities and keep things civil.
facebook, twitter, and tumbler all got along without community moderation, i don't see how reddit would actually be any different. every subreddit is a glorified hashtag in the grand scheme of things.