I tend to only upvote comments that add value/content to the blog entry and community (clarifications, error corrections, pointers towards other relevant content, ...)
A mere, "thank you, great post!" or even the lengthier versions don't get my upvote. This is my personal opinion but I have noted most people reason like this. This is not facebook. Our vote is worth something to the votee. I rather spend my vote on a good blog entry than on a contentless comment.