Biggest difference for me betweeen HN/reddit and the forums of yore is how the ranking/sorting is done. On HN/reddit, "most popular" opinion or "best sounding" post usually "wins" and gets most discussed, as it's at the top of the page.
Meanwhile, forums doesn't re-order things like that (didn't used to at least), you made a post and it ended up after the message posted before you, and before messages posted after. Everyone's view and message was equal, so pile-ons or hive-mind "this is the right way of thinking" seemed less common.