and what currency that would be! make some successful shit posts and it gives you the privilege of silencing a political opponent in a debate! great fun
For an example implementation, downvoting in stackexchange costs you 1 point and takes 2 from the person being downvoted iirc (an upvote gets you nothing and bestows 10 points). You can only downvote if you've got a few points on the site, many fewer than on HN (the threshold here is 500 iirc). Flagging is always possible but afaik never leads to automatically killing the thing you're flagging, so downvoting is the way to downrank and fade out posts of people you disagree with. If you think this isn't a good system, it might be worth looking at how it works there, what the problems and benefits are and make a suggestion of what would be better