I definitely feel you here. This is a trend I see absolutely everywhere; hell, I was surprised to find out it even extends into video games that implement this kind of voting system.
This problem feels like it could be solvable by changing the "meaning" of the upvote. Right now, the upvote is a tool that allows you to make a comment more visible and to throw a bone to the author in the form of karma. People probably feel inclined to "tip" someone who makes a clever or witty comment with the karma from the upvote. On top of that, people making witty comments who get upvoted are now encouraged to make more witty and unproductive comments because they know that they will get karma for it.
The issue is that the upvote represents general approval of a comment, rather than a nod of the head that a comment is productive or insightful.
Just throwing around ideas - maybe the elimination of the user-karma-tracking system altogether is a reasonable solution?