In this particular post, I'm upvoting all the comments which make me think. So if I agree with someone's post, but it's pretty much a cached-thought for me, I won't bother upvoting it. And if I strongly disagree with someone, but they've forced me to think about why I disagree with them, I upvote it.

(This isn't the metric I normally use for deciding when to upvote in other LW posts.)