I personally, think the downvote activation should be a combination of length of membership and karma. It does seem that as of late, there has been an increase in "I don't agree with you, so down you go".
Anyway, I am getting off on a tangent. There are a good deal of developers who disagree with this, some instead of forming a rebuttal use the down arrow as their rebuttal.
For me personally I see it, when used in that context, as a "I don't have a strong rebuttal, so I will try to make you post go away" line of reasoning. I had one guy get so annoyed at me, that he went in and bombed me on any post that he and I did not have an exchange on (you can not down vote if you reply to a post).
To me those kinds of actions are site killers, fortunately for HN, it has not reached a critical mass, and it still has a good deal of intellectuals looking for good conversation. I have just chalked it up to, oh well you take you licks. At the point that all of my post get downvoted then I will know that I have been voted off the island, and that it is time for me to leave, which is fine as the community will not be representative of the people I am looking to interact with. As of yet that is not the case, so I just go with the flow.