I would abstain from complicating the user experience, though. In my experience, posting a topic has much more potential in gaining points than writing *thoughtful* comments. A good strategy to rise above that ranks is to post a bunch of interesting links; upon achieving high points, the user can then take a defensive stance by defending his or her opinions through posting comments instead.
The point is, from a user's perspective, there is more to gain from posting topics than comments.
That is certainly the status quo; I would ask, however, whether it is desirable. I think the system should provide more incentive for thoughtful comments, or at least some sort of disincentive for shotgun-style submissions. It is difficult to build karma by posting lots of comments; you have to put the effort into generating good content to get karma. The same should ideally be true for submissions; the difficulty, of course, is that since the content of submissions is generated by someone other than the submitter, simply posting lots of them /will/ eventually result in a karma-boosting hit.
We had a similar problem on our social network where users abstained from giving karma to thoughtful comments. Consequently, we coded an algo to add "+0.1 karma" to every user that gave another person "+1 karma". That was enough to take care of the problem.
The point is, from a user's perspective, there is more to gain from posting topics than comments.