Perhaps the length of the comment text can be factored into this heuristic. I mentally correlate lengthy posts as tending to be well thought out and coming from people with experience in the given area.

While a lot of insight can indeed fit into a sentence or two, it still figures to me that those who take the time to write long comments are probably contributing more to the thread than those who leave short replies.

Anyhow, it's just a heuristic that springs to mind.