Comparing Imgur to HN is comparing apples to oranges in my opinion, they have different purposes and target audiences.
Target audiences:
Imgur - General public.
HN - Technical people, hackers, business people, founders.
Purposes:
Imgur - People submit funny/touching/stupid/whatever images and people post comments
HN - People post interesting/thought-provoking articles/sites and discuss them.
Target audiences:
Purposes: