On a related note, the community's voting patterns encourage the witty one-line comments over the thoughtful 3-sentence comments. Look at any article, and you'll see the type of comments this incentive produces.
It's hard to represent something subjective in a single number. This problem isn't really solvable. Making the karma algorithm too complex could make it even worse.
I think making someone's karma filter subjective could solve the problem. Then it would also be hard to game the system, since you'd have to know how most people filtered karma.
This filter could either be based on how people vote, or using karma and something else as the criteria.
An example of the latter, I've thought the good comments would be those that maximize length and karma. That means they are both substantive and well written enough to hold people's attention. Plus, it's too long to be witty:) This could be implemented entirely client side without the need to open up voting data.
Finally, it'd fit great into the whole hacker mentality, since people could pass around and hack the best filters.
(note: this idea has been said by many others, I'm merely trying to perpetuate the meme)
Although I'm guilty of making an occasional witty one liner, I generally down-mod them from the outset to try to discourage them. It seems like some others do as well.
