Earlier quoted context omitted.
One problem with posts costing karma is that it would all but guarantee to stifle long discussions amongst any but the highest karma members -- especially if you end up autobanned when you run out.
That could be a good thing. It's very rare to see a deeply nested thread actually worth reading.
I know pg's added things like a progressive delay to the reply box further into threads. The curve of quality going into a thread is an interesting problem, I can see how the subjects of discussion would go from general to specific (and thus likely less interesting to general readers) the further down branches you go, but also sort of by definition those branches become more and more relevant to the people involved. Maybe a flat or hybrid flat/tree layout would help keep discussions more linear?
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.)