* they demand that discussion be evidence-based
I don't think that's strange, and I think it's actually slightly less evident on HN that on many other sites.
I think that in internet discussions, a lot of assertions tend to be challenged due to the relative anonymity and high occurrence of counter-factual statements. This makes it hard to have opinion based or exploratory discussions, as any random thought you put forwards, however useful, can easily generate a "please show references" thread.
However, in a place where people can demonstrate domain knowledge, expertise, and also trustworthiness, people can take your word and only ask for follow-up out of interests sake. For example, if grellas commented on a law topic or tptacek commented on security then most would not challenge them.
In relation to the seriousness, I think that people here are quite respectful (relative to the rest of the internet) of other's time. So, they don't joke around so much, I guess. I think maybe that is unusual, another word for that is strange.