I've observed that simple, positive feedback like "Looks good, keep it up" gets downvoted as nonconstructive. Karma is more than internet points here since the thresholds drive so many features, so I bet people who would otherwise comment in the affirmative simply don't. Paradoxically, nit picking and verbose argument can score a lot.

I don't want to offer any particular suggestion because I don't have any data to support anything. A hunch, though, based on your idea: if someone want to downvote, a comment must be provided.