The problem is that commenting in posts that are several hours old, is penalized. If you write a really interesting comment, but the post is about to drop off

The problem is that commenting in posts that are several hours old, is penalized. If you write a really interesting comment, but the post is about to drop off the main page, you're karma score will be penalized (because no one read your comment). Absolute karma score is a measure of how many people like your comment out of how many people have read it. There's just no way to determine how many people have read it. One solution would be to compare how much total karma the comments in the post have accumulated when you post. Then compare it to how much karma there is once reading has stopped.