But the more I ponder what you're trying to solve, the more I think you should provide scores weighted within the discussion. Perhaps, the quintile a comment is in. Use a bar that adds segments and shifts from one non-judgmental color to another. I wouldn't bother showing anything below a threshold of ten comments on a single link.
Otherwise, your piling-on problem returns, just with a little less fidelity.
On second thought, the problem with the quintile idea is that it presumes that every thread has an equal distribution of valuable contributions, middling contributions, and lousy contributions. But we don't want to rule out (e.g.) the possibility that an entire thread might be great and that every post deserves a dot.
Moreover, the quintile idea retains aspects of the piling-on problem: A sufficient quantity of super-upvoted comments will push other worthy comments out of the top quintiles.
Finally, quintiles are just way too complicated.
So I'm prepared to believe that I was wrong twelve hours ago. I like the threshold idea. Especially because the threshold can be tuned. If 5 isn't enough it can be turned up to 11 by a well-meaning moderator, acting at the appropriate time.