Clearly a very subjective topic so work with me here...(and forgive my clumsy explanation).
Comparing hn to other discussion boards I'd say it has far higher quality discussions yet I can see no obvious signs of moderation. (Or even aggressive algos like reddit).
Responses are generally well reasoned, low on personal attack & other undesirable traits. Criticising people frequently results in introspection on their part rather than blind retribution.
All this is great of course...yet leaves me a little confused as to what the magic behind the curtains is. What's different vs other discussion boards?
HN also forbids the discussion of what CNN considers news. We don't have endless knock-down-drag-outs about the usual subjects.
Also HN has a fairly unique community of people who are interested in both technology and business, no matter where they are located.