If I may paraphrase Anonymonk's suggestion:

The voting system is currently being used for two orthogonal purposes: To indicate Node quality and to indicate agreement/disagreement.

I agree with the suggestion that dinging someone's XP if you simply disagree is not conducive to discussion.

As a solution, I propose adding a SURVEY OPTION in a node - that allows the author to add another set of radio buttons that indicate the degree of the reader's agreement with the author. Perhaps even multiple survey options, in case the author wants feedback on several issues.