The bigger problem with reddit, in my experience, is that 95% of the comments on any given post are pure garbage filler. Joke chains as far as the eye can see.
• 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
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.
For me, as with HN, I don't like the inherent stress of commenting on submissions before they fall off the front page. The earlier one comments, the higher the chances of getting eyeballs/responses. After that short ~24 hour window, people may still read the comments, but submitting new comments seems futile.
As opposed to older "style" forums, even today, threads often stay open, allowing people to post months or years later to revive "dead" threads.
I'm sure this approach also eases the moderation burden, and I'm pretty impressed overall with how dang handles things. So maybe it is a simple workload necessity. It does strike me that HN could be quite a bit more if these two restrictions were loosed.