A major culprit there is the absence of private messaging. There's no way to build intimacy. Sure, you can email, but that's a much larger step change in interaction - and giving your email to one person means giving it to everyone.
It may also (unintentionally?) serve to limit strong bonds developing on the site and prevent the rise of well-known posters who aren't already HN celebrities for reasons outside their posting on the site, reducing the likelihood or effectiveness of offshoots or schisms that have been a common feature of other large Web forums.
For a long time I resisted the pull of avatars, and thought of them as gimmicky. (This coming from the mailing list / Usenet tradition in large part.)
But having used forums in which avatars are and are not used (G+, Diaspora, and Mastadon amongst the former, HN, Reddit, and Tildes amongst the latter), I feel a palpable difference between the two. Most of us are sufficiently visually-oriented to pick up on a symbolic representation (avatars need not be photographs or faces), and there's a more rapid assimilation of these than there is of just a textual tag.
I've also noticed on platforms / cultures in which changing the visible handle / avatar combination is fairly common (this seems to be the case on Mastodon especially) that this is disruptive. Yes, there's a persistent user name, but changing the associated image and handle ... breaks that recognition. On balance I rather dislike it.
I'd had a brief experience on a site in which serial pseudonymity and a lack of long-term persistent identity was a norm (Imzy). That I think was one of several ultimately fatal mechanisms for the platform --- it was shut down after about a year.
But on old forums there was also the fact that the communities were so small that just hanging out a bit lets you get to know all the regulars. The closest I get to that nowadays is probably guilds in MMOs. There's also Discord, but that's a constant rolling conversation which reminds me of IRC, which was never really my cup of tea.