My thought is downvote, but it will take away from your karma (-1 point). The only way it wouldn't or you get that point back is if you left a comment, possibly responding why you downvoted in the first place (factual, wrong, not relevant to the discussion, etc). Quid pro quo.

I think putting "real" value to karma would be a bad idea. People already karma whore even if it has no real value (although it has perceived value). I imagine it would be worse if it has some actual (limited) value.

How would you combat spamming or obvious admissions of astroturfing? Seems like it would be easy to game the system by bombarding it with "shit posts."

Hmm, how about one user downvotes, provides the appropriate reason, then others can "join or agree" with that user's downvote by upvoting his/her comment. Maybe this could somehow count as multiple downvotes with the same reason?

Also anyone who downvoted without commenting should have their hn-name displayed on the right side or below in a list of silent downvoters. It should of course adapt if you comment later or remove your comment, by removing or adding your name to the downvoters list.