Give the user their upmod points of the entire thread that responds.
That sounds like a surefire way to encourage provocation (a.k.a trolling). If you make a deliberately provocative and clueless statement and n people take the time to exhaustively tell you why you're wrong (for which they get karma), I don't think you deserve any of the karma given to the comments you provoked.
Only give people the upmod points of responders if the original comment has positive karma. Or let's say your original comment has karma +4; you get karma of responders up to +4 per response, but not more than that.
The more complicated the system, the easier it is to game.
Presumably at some point that ceases to be true for a sufficiently complicated and good system, but my guess is that we're up into AI-complete territory there.
That sounds like a surefire way to encourage provocation (a.k.a trolling). If you make a deliberately provocative and clueless statement and n people take the time to exhaustively tell you why you're wrong (for which they get karma), I don't think you deserve any of the karma given to the comments you provoked.