The problem with this is firstly that it creates a feedback loop, where the more commented something is, the higher it climbs, the more commented it is...
Secondly, it'd be hard to extract a quality measure from comments alone - there are a number of metrics (number of unique users commenting, rate of commenting, amount of voting...) but how do you get from that to a quality measure? How do you distinguish controversy from quality? Or comment threads full or jokes and snark?
Also, commenting on anything you disagree with or dislike would result in that link getting a boost, which can't be right.
All that said, I think comments could be used, but modestly, as part of a bigger algorithm.