There already are certain privileges to karma: you need a certain threshold to downvote comments at all, and I suspect that thresholds to get onto the front page are based on the karma of who upvotes your story. (There've been times where my upvote alone, going from 2 to 3 votes, is enough to get something on the front page, while many times I've seen new stories with 4-5 points that don't make it.) I suspect PG is thinking of tweaking the weights, automatically, until there's enough of a weight to high-karma downvotes to kill bad comments. You could easily train an SVM or other classifier on some set good/bad comments and then pull out the coefficients to figure out what signals should go into weighting point totals.