HN is also quite bad to have long-term discussions about a topic, and suffers from the same constant rehashing of topics happening on reddit. It sort of works for news and random individual articles, where there aren't necessarily longer logical threads, but that's not what many communities need.
E.g. a typical comparison between forums and reddit: A hobby forum often has a "I just bought X" megathread or two, where people post new things they've bought and want to share their excitement about, but that don't warrant a full thread. On hobby subreddits, a large amount of the threads can be "I just bought the thing everyone always recommends and everyone has seen 20 times this week, can't wait to use it!". Similarly, what would have been a single post on an old thread in a forum needs to be it's own thread on reddit, loosing context and making search harder (even if people try to link other relevant threads). Which in turn leads to more repeat questions etc.
Hacker news is 30x worse because you don't even see replies to your comments so the fire and forget mentality is very strong. Also very discouraging to reply to older comments because once it gets over a day old its likely even the person you reply to wont see it.
• Discourages long-running discussions by burying (and ultimately locking) older content
• Discourages coversation in favour of fire-and-forget comments by making it impractical to continue where you left off
• Gamification of content (not quite as bad as some sites that grant extra privileges for scores, though)
• Numerically larger groups can bury content they would prefer others not see