In principle, I suppose there could be multi-dimensional voting, with at least different dimensions for degree of agreement, for how well-argued a comment is, and for degree of relevance to the topic (or at least sub-thread). Of course, if one goes far enough down that road just choosing the multidimensional vote starts to become an energy drain in and of itself... (www.ted.com has at least 8 dimensions for rating their talks - which is enough to dissuade me from rating them...)