They have all sorts of modern features more conducive to discussion and community-building like notifications that someone @mention/replied to you and even editing your post -- features that people generally like. If you don't think that's "fun", fair enough, but I also enumerated other benefits like their broader appeal.

Any community that only has a mailing list could benefit from experimenting with a proper forum. I've seen this experiment broaden a community time and time again as you move away from only selecting for the type of person who likes mailing lists. And notice that HN isn't a mailing list either.

For example, I would imagine that the sort of people interested in high-quality ebooks extend beyond mailing list loving super-techies. Even a subreddit would be a nice option.