Because there's a karma threshold attached to promotion, a user who has some demonstrated engagement with the site will have to make a deliberate decision to grant a forum to limbo comments. That person will have to believe the comment makes a positive contribution and maintains civility.
The phenomenon that bolts fatuous vitriol to the tops of threads requires no such deliberation.
I have no idea if this is going to work, but I'm psyched to see it play out.
In another thread not long ago PG stated that this was exactly the case on a few comments he found vitriolic. If he measured the votes of only high-karma users, the comments would have been negative. You can search through his history to find his report on the subject (it was around three weeks ago, IIRC).
To be clear: what he's saying in this thread seems to support the idea of a karma-limited feature for showing comments. The score (not the content) on a comment would have been negative; the comment, in context, was a superficial negative comment that commanded the top spot on a thread.
At the very least, the intuition seems to me that if you have a high enough karma score, you've learned by example what is an acceptable and unacceptable comment and know how to compose comments that are acceptable, and should be able to tell the difference. Beyond that, if you set the knob high enough, you're going to capture people who have been around for a while and have been able to witness the rise of the pendants and armchair-experts compared to yesteryear so won't dismiss these things as "normal."
If this is the case, a good optimisation might be to weight the value of a vote according to the karma of the person casting that vote. This seems like a simple idea to me, and I don't know if it's used elsewhere, so please forgive me if I'm being a bit naive here!
The phenomenon that bolts fatuous vitriol to the tops of threads requires no such deliberation.
I have no idea if this is going to work, but I'm psyched to see it play out.