When you have a new comment notification, you are more likely to respond. People are more confrontational on reddit because of this reason. Whereas on HN, you tend to write a thoughtful answer and leave it at that. Threads are shorter on average, so the conversation is less likely to get out of topic.
Collapsed comments are usually ignored, because people are too lazy to dig that deep, that means some really good conversation might get buried.
I kinda like HN comments style better, I hope they keep it like that.