What's right and wrong is rarely as clear cut as that when you're talking about the type of topics discussed here. At any rate I'm not talking about rewarding well-written but clearly wrong posts, but rather posts that are interesting but of uncertain truth value (which is a lot of them on this site). Agree/disagree on the other hand would allow people to register whether they believe something is true/a good idea independently of how interesting it is.

I guess rather than having two ratings that could be given seperately, you could vote a post as "agree" or "interesting", both giving postive karma, or "disagree" or "bad", both giving negative karma, but with visual distinguishment. This would give posters more accurate feedback without fundamentally changing the karma system.

Right now if a post has a high rating it's unclear if people actually agree with it/think it's true or just find it useful/interesting, and similarly for low ratings.