x is actually 501.
Even past that threshold, you can't downvote responses to your own comments. That's probably a wise limitation when you think about it.
Downvoting threads is done with the "flag" button which appears at the same time. Unfortunately, as HN has grown past a certain size, there's gotten to be a critical mass of "made" high-karma censors, who flag nascent postings on a whim. The diversity of articles suffers, limited now to either the things censors like, plus a small selection of postings where teams will "unflag" (also available at 501) this price-of-admission flagging.
Running any kind of good forum is hard, and HN's moderators work really hard at it, but "strangled-in-the-crib" flagging both takes away from the value of the site and gamifies the forum to where it now takes a team to make a posting that survives. I don't have an easy solution, though it might be worth trying to limit flags to one, two, or ten per day.
