Your comment suggests an interesting way of rewarding people who further the conversation. Give the user their upmod points of the entire thread that responds.
1 - This makes the witty comeback a lesser strategy for obtaining karma.
2 - People who are aware of this will stop responding to trolls. Even if they publish a well-reasoned response to a troll, they will be giving them karma points. Since this is not a common feature of internet forums, this might be worth spelling out above an active comment form.
3 - The best strategy for karma is to advance the discussion somehow. Even if your point ends up exposing the point of view of people on the other side.
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.
Problem with this system is that it further skews the rewards to early commenters, especially those that play devil's advocate. People who post early already have an advantage since there is less competition for upvotes, and newer comments appear below slightly upvoted comments by default, ensuring better visibility for early upvoted comments. Adding you suggested karma rule would make this even worse.
1 - This makes the witty comeback a lesser strategy for obtaining karma.
2 - People who are aware of this will stop responding to trolls. Even if they publish a well-reasoned response to a troll, they will be giving them karma points. Since this is not a common feature of internet forums, this might be worth spelling out above an active comment form.
3 - The best strategy for karma is to advance the discussion somehow. Even if your point ends up exposing the point of view of people on the other side.