I don't think it's possible to build a system without human intervention that keeps the bad content out and promotes interesting discussions. What I'm essentially saying is that the Reddit staff should pick a few thousand people by hand that they think embodies what they want the perfect redditor to be like. Everybody else starts out with minuscule influence. Now those ambassadors will go out and up-vote other good users and down-vote bad ones and will in that way spread their influence to other people. Some users might even have negative influence. Can you imagine a user who keeps up-voting cat pictures that is in effect down-voting those submissions without realizing it? It would be beautiful. Obviously it would be important to keep users' influence a secret.
In essence, it would give the Reddit staff much needed control of the content and discourse. If they one day wanted to steer the site towards only cat pictures, they can easily do that by selecting those users to be the ambassadors.
Right now it seems that Reddit is suffering from Eternal September x 1000, and they're at the mercy of the masses. But the geeky, thoughtful content that brought people to the site to begin with is nowhere to be found anymore. It's being drowned in memes and cat pictures.