I quite like the idea that a downvote triggers the necessity to leave a comment justifying that downvote. If someone feels that 'the post doesn't deserve the rewards it's currently getting', then it would be helpful to know why an individual feels this way. It may also act in a way that discourages that downvote if it's simply a reactive action based on personal bias.
That idea has been discussed before, but there's a lot of ways around it, and it doesn't ultimately stop the problem.