i don't know. The more i look at hn the more i appreciate the thought that went into it. Stories are far more black and white- either they violate the guidelines or not. The only thing a downvote would do is to make the story float all over the front page as there is an inevitable vote war. I would even say that most users don't like any given news story, which would lead to the situation that everything would get downvoted into oblivion. Comments OTOH lend themselves far more to moderation by downvotes. Remember we still get the "nuclear option" of flagging a submission.