Voting up fails to distinguish between the overall point, the individual arguments, the novelty of the point made, and other possible factors.

There's a big difference between "I agree with your entire point", "I agree with most of your arguments, but I dispute point 3 which may undermine the point being made", and "I think you've made a novel and plausible point, but I disagree with it", all of which are (for me at least) valid reasons to upvote something.

Simply voting up and commenting makes it unclear which of the above, if any, a given commenter feels.