Incidentally, I often get unknown or expired link when I submit comments, which is presumably HackerNews' way of saying that I took too long considering and drafting my comment. This seems to discourage thoughtful replies.

It seems to me that intellectually stimulating discussion would be served better by a longer timeout.

(fortunately, the back-button and cut-and-paste circumvents this problem, but it gave me a start the first time).