More often then not, when that threshold is crossed, I realize I'm too late and my post will just sit there, unread.
Aren't there others who think it'd be great to have a "sticky" or "follow" button, in essence signalling "I'm going to follow this discussion for a few days / weeks, because I'm genuinely interested"?
Also if there was a way to easily "suscribe" to a thread (email alerts) that'd help. (Someone mentioned hnreplies.com in a sibling comment, never tried it, but I'll check it out)