Such a system would mostly add ceremony, complexity and meta without obvious benefit to conversation interestingness. I think the other thing is a more fruitful line of inquiry - how come nobody wants notes on upvotes? The answer, I think, is getting downvoted feels bad and people naturally want more friction, 'accountability', etc when something makes them feel bad. However natural, though, this isn't a great way to design forum systems - it's more elabourate rationalization than design. The question of whether a particular forum's negative feedback system is too punishing, imbalanced, makes users feel too bad too easily or too soon is a perfectly sensible one and worth exploring, of course.