HN hides the reply button on some comments that it deems might start flame wars (or similar). If you click the link link, that will let you reply from there.
The "reply" link is also hidden on comments posted less than some (0 < n < 9) number of minutes prior to page load, but the same trick works in that case as well.
aaronem|12 years ago
jneen|12 years ago