I think there's an implicit conflict of interest between the features and focus of a "social" site, and quality of content. Appealing to the lowest common demoninator will drive up your status (views, karma, XP, whatever) faster than content which invites debate and possibly criticism. I noticed the urls for the stories on this site branch out from user accounts too -- to me that's a bit of a red flag that whatever passes for karma-whoring over there will be an issue, since there's a direct association between story, user and user status.

I don't know how you would engineer actively rejects junk content, if you even could - it seems to me having a site where users post content means a community has to be reactive about curating it. Maybe focusing on content groups rather than user groups or having it be invite-only. But still, what you want lies mostly in the eye of the beholder.