This is too weird. I was typing up a similar question today, wondering if structure forums are still superior in this age of Reddit-style websites.
Forums allow sophisticated discussions to occur for longer than 48 hours. My biggest complaint with HN is that discussions always terminate themselves shortly after the article moves from the frontpage. Its just too difficult to track new comments that aren't directly replied to your own.
I'm a big fan of serious hobby forums. Places like HardForum or the late TheGreenButton have lots of in-depth, long-standing, technical discussions. Discussions that just wouldn't be feasible on another software platform.