I find it's mostly that posting a comment is very different from entering into discussion. There is a surprising amount of friction attached to entering into a discussion on the web, compared to just lobbing a comment onto a page (like I'm doing right now). Throw in point scoring to comments, and you've created a system that just doesn't appeal to the majority of people.
You get publicly scored on your contributions to the discussion. Most people are turned off by the idea of discussions being adversarial, point scoring, confrontational.
And in that respect, I consider the barriers to comment contribution to be very high indeed.