Why? I don't think there's a problem of too much downvoting. As far as I can tell, the comment quality has actually improved recently with better comments bubbling to the top while not-as-constructive ones fading away.

If anything, link submissions or comment submissions should deduct karma so as to curb spammy (and possibly unsubstantiated) behavior. Of course this lends itself to issues of new users not being able to engage as much.