I think having 3 options would help with this problem: agree, disagree, and downmod. Top agreed posts would float to the top as they do now. Even posts with a healthy bit of disagreement (evenly matched agrees/disagrees) could be very high. Disagreed posts aren't penalized. Downmod would only be used for irrelevant or low quality content. You wouldn't need an upmod -- simply agreeing or disagreeing would count as an upmod.