I tend to agree that using a down vote to express disagreement eventually isn't constructive. I've seen a number of posts that didn't say anything objectionable or controversial, nor were meaningless in the context of the discussion, and yet were down modded.
My theory is that while the risk of being down modded on HN is a helpful factor in keeping discussions more courteous than on an out-and-out anonymous forum, the same cannot be said of down votes, which are in fact anonymous. As a result, users tend to be more careless or perhaps even callous in the use of down votes. I'm not sure what the solution is though: making down votes transparent might help in creating a bit more discreetness in their use, but might spill things over into flame wars into the comments themselves when exercised. Still, I think it's the total anonymity that causes its over use.
My theory is that while the risk of being down modded on HN is a helpful factor in keeping discussions more courteous than on an out-and-out anonymous forum, the same cannot be said of down votes, which are in fact anonymous. As a result, users tend to be more careless or perhaps even callous in the use of down votes. I'm not sure what the solution is though: making down votes transparent might help in creating a bit more discreetness in their use, but might spill things over into flame wars into the comments themselves when exercised. Still, I think it's the total anonymity that causes its over use.