I agree that the main appeal of HN is undoubtedly the community. There's no other place where you can be exposed to so many no-bullshit comments and submissions from so many people so deeply embedded in the industry.
However, I disagree that the design has nothing to do with it. I think the spartan design keeps the appeal of HN low for those who don't fully appreciate the value the community itself brings. Case in point, many people who post here are also probably on Twitter and/or Facebook. But Twitter and FB knowingly pursue such directions in their UI and UX and overall service design that having anything like HN on those platforms is all but impossible.
Hmm... these things are hard to tell. I’ll give you my personal motivation to stay here and that’s unquestionably the design. I never got hooked on Slashdot because that site was an eye sore (sorry!).
Usually, good design fades in the background when it’s good. So we give the credit to other things.
What this website looks like, how fast it loads, and how simple it is are not reasons to post here.
With the emphasis on 'here', I think they are very important factors.
What this website looks like, how fast it loads, and how simple it is are not reasons to post here.
People post to contribute their thoughts to the community. There are people here who we want to discuss things with. There are topics we're interested in that we have opinions about. We each want to be seen to be curious or knowledgeable about tech things.
The design of HN is good because it lowers the barrier to posting. It's simple and obvious how to use it[1]. Those are good things but they're not enough to keep people here. That comes from the links and the comments.
Good design isn't enough to make people here stay on a website. Bad design is enough for people to leave.
[1] Apart from all the stuff new users don't understand like the black bar, the username colors, why comments turn grey, why you can or can't vote etc. Those bits are less good design.