At the end of the day, for me, it comes down to the content when considering why I visit the site. HN constantly has interesting, engaging content, and for the most part, insightful discussions. Although, it can get pretty pedantic at times. Where else can one go for this? Reddit involves sifting through multiple subreddits to get the type of content that's on here, which can be tiresome.
Regarding the interface, I love the site's design. No matter what kind of connection I'm on I can browse HN, read comments, etc without the usual modern web clutter. In my opinion, there's no need for a messaging system either (just another site to check over and over) - just have your email in your bio. Easy.