This is actually a pretty bad idea because it would discourage people from commenting on anything but the top posts on the first page. Commenting on something

This is actually a pretty bad idea because it would discourage people from commenting on anything but the top posts on the first page. Commenting on something interesting that's 3 pages deep would basically mean throwing away karma unless that post manages to get up-voted, which is never a sure thing. Of course, some good discussion is often what gets good material upvoted! This change, while it might improve comment quality, would drastically impair the upvoting process.