But why shouldn't it correlate with post count? That way the incentive structure encourages active participation; under the system Eliezer prefers, people might be tempted to hold back.

Since comments tend to get upvoted more often than downvoted, why would people hold back? A comment's 'expected karma' would still be positive :)

We can still add an extra "post count" to a user's profile if anybody needs to know that.

And no, I don't think that this site is glutted with low-quality comments, but it's young and I already can't keep track of all that's being said, so a gentle nudge in the "quality over quantity" direction would be a Good Thing.