I agree with the above convo you've been having, but your comment made me think, so I'll play a little devils advocate here: do we want 1000 "I agree." comments following each other comment? Not really. There needs to be a simple way to say "I like this but don't have anything to add". An upvote accomplishes this.
As for downvotes, yes they need to be more than "I disagree". Something akin to "I don't think this contributes". I liked someone's suggestion that we need more than the binary up and down. Maybe a "troll" vote too.
I caught the joke :) I mentioned playing devils advocate cus I don't want to actually attack you or anything, I just saw an interesting opposing viewpoint that I hadn't heard expressed yet. I think your closer to the best solution in general tho