1. If, now or in the future, the HN comment-ordering algorithm pays attention to children's scores, then you get the following pathology: A says something slightly wrong, B makes a small correction, B's comment gets massively upvoted because everyone upvotes it, so that subthread gets treated as highly-scored even though there's nothing terribly insightful or interesting about it. (You could, but probably wouldn't, get the following even worse follow-on pathology: people start putting mistakes into their comments so that this will happen and they'll get extra visibility.)
2. People get more karma for posting minor corrections than for posting substantive comments. That's arguably unfair, and again it has a possible follow-on pathology: making nitpicking more popular and carefully thought-out insightful comments less popular.