A submitted article has high visibility and has no risk to karma. Contributing to a discussion has low visibility and risks karma. An RSS reader and a bookmarklet makes article submission semi-automated but insightful discussion takes more effort. Perhaps there should be more points for discussion.
I'm sorry, what problem are you solving here? The problem of a new user writing a bot and hogging the karma? While allowing an "old hand" to do exactly the same thing and reap the rewards?
Anyway, the reward of getting from karma 1 to karma R is greater than from R to R + hogging. If a user is past R karma and then decides to hog, then they're a moron and we should do something about that too.
No ideas, for the simple reason that I have a basic pessimism about trying to use carrots and sticks to create behaviour that is beneficial to a social group.
"We don't teach people to be nice, we hire nice people" --Leona Helmsley (sp?)
I think we can punish egregiously bad behaviour and we can moderate things, that eliminates outliers. But fundamentally our approach should be to encourage nice people to join and discourage trolls so that they leave.
It is not clear to me that karma is a good mechanism for that... if there is a game, some people will play for the sheer pleasure of winning.
I think the number one way to encourage nice people to use a social site responsibly is to swiftly and severly punish trolling. Nothing sends a message that you prefer signal over noise like banning noisemakers.
(Q = R = 30, S = 0.1)