My idea is, that when people read something, and they agree/disagree with it, they want to express their agreement/disagreement (this is covered in this thread). When you want to express disagreement, you have to, because it's no downvote here, comment to disagree with some opinon. But when you want express agreement, and you don't want to comment (or simple 'I agree' wouldn't add anything to the debate), you just hit the upvote button. And that's problem, as the Vote button should serve as "quality content here" indicator, not "popular content" indicator. We've seen it on Reddit, where the effect is even larger because of the downvote button.
So my idea is - Add agree/disagree buttons. These buttons doesn't have to do something - I even think, that it'll be better if they didn't do anything. But they'll help users express themselves, so when someone agrees with me, but doesn't want to simply say "I agree", he can hit the 'I agree' button - and he won't hit the vote button. This way, popularity won't affect the "quality" score of the post.
Edit: As @Kat, @Kijafa, and others said, maybe add only agree button - not the disagree one. I think they might be right, disagree button might have negative effect, as now users have to express disagreement via comments.
Edit 2: Thank you all for discussion. You made me change my mind about disagree buttons, you are right - it'd be used as downvote and would probably suppress discussion. However, I'm still curios about the "I agree" button and if it would decrease of usage Vote button to express agreement.