[um_ya]

I think it comes down to group size and subject matter which drives meaningful debate.

When group size is large, conversation is shallow.

HN is a relatively small group with a relatively confined subject matter.

Same concept applies on reddit, with small niche subreddits.

There's more conversation depth with smaller groups of people.

[ketzo]

I mostly agree with you, but I think HN is definitely a special case. It's not huge, sure, but it's certainly not small - I think there's a very active moderation effort and community spirit that keeps discussion much more productive than your average internet forum.