So the likely reason that HN does not allow Markdown is because there is/was no Arc implementation of Markdown.
https://news.ycombinator.com/formatdoc
Esp. the "no links in the text-field, but in the comments"
It is successful :-)
pg knows
It would be easy to implement some more primitives (and make them translate into HTML)
It MUST be intensional
(1) He had very little time
(2) Markdown was just one potentially easy feature among many other potentially easy feature.
(3) As he stated many times, his focus was on reducing the amount of harmful comments.
What would you use, and why?