The parent comment says X, the one below says the opposite, and then someone says X again. Do people click and write mainly to get karma? Should posting also cost karma?
The longer you write the easier it is to say something that's not true and harder for people to follow it accurately.
(I know I'm guilty of this.)
But...I do agree with you. It becomes harder to sift through facts when a post is very long. I do it because I enjoy writing long prose on topics I'm interested in - I don't think it's particularly correlated with getting high karma. I've seen very high comments that consist of a little paragraph (albeit packed with technical information).
But I think a lot of people do just click and write for karma. As long as there is a karma system, this is somewhat unavoidable. I really wish we could do away with the entire karma system entirely, but your suggestion about posts "costing" karma sounds really neat, I'd definitely test that on a small forum...not sure how you'd deal with throwaways though, and how would new users accrue karma?
I don't know if giving that much more power to older posters is necessarily the answer, although it might help reinforce the perception of the community maintaining a certain tone in discussion, if the same posters are more likely to be heard and heard more often. On the other hand, with that scenario, karma would actually mean something (though that just brings up the possibility of karma-farming posts.)