If you see good comments that are dead, you should vouch for them by clicking the 'vouch' link that appears next to the 'flag' link. To see those links, you need to go to the individual page for the comment, which you can do by clicking on its timestamp. There's a small karma threshold (currently 30) before those links appear.

When enough users vouch for a comment, HN's software unkills it. In addition, we review all such comments, and in cases where an account has been following the HN guidelines for sufficiently long, we'll unban the account. Thus this mechanism has become a way not only for good comments to be rescued, but for reformed accounts to be rehabilitated.

Occasionally we notice an account that was banned by mistake, too—usually by a spam filter that hit them when the account was brand new—and those we unban right away. So vouching for comments has turned into an effective way to help your fellow users.

But please be conservative with vouches. If a comment breaks the HN guidelines, or skirts close to the border, it shouldn't be rescued. Otherwise there's too much potential for abusing the feature.