Although I have not been on here long, I have to say that I agree that votes should be cast with reason, good or bad.

Even if it's something as simple as, "Thank you, very useful post."

A drop-down, even, with options to choose from:

Vote +1: Excellent Post!
Vote +1: Very Useful!
Vote -1: I disagree.
Vote -1: Drama!

Of course, those are just off the top of my head, I'm sure others could do better.

That said, I am not sure that negative voting is a good thing. Why give see more someone a chance to tear others down? Instead, if a post is truly not useful, allow a "report button," and if the post is reported too many times, it is looked at for moderation.

Again, just throwing out ideas. Not perfect ones, just ways to look at the situation. *smiles*