In my experience (And observations), depending on _interest_ posts start gathering comments after at least the hour mark. This post right now is 25 mins old, although its' weird that it's so high in the front page (Probably a slow Saturday)
How are stories ranked?
The basic algorithm divides points by a power of the time since a story was submitted. Comments in threads are ranked the same way.
Other factors affecting rank include user flags, anti-abuse software, software which demotes overheated discussions, account or site weighting, and moderator action.
(To be clear, I'm not implying nor do I think that HN uses raw comment-count as a signal of overheating, just expressing my surprise at the previous poster's surprise at seeing high-ranked stories with few comments, since the correlation should run the other way if at all.)