Hacker News is written in Arc, a dialect of Lisp: http://en.wikipedia.org/wiki/Arc_(programming_language)

Reddit used be written in Lisp, but they switched to Python early on: http://blog.reddit.com/2005/12/on-lisp.html

I do agree that they look alike, Hacker News was inspired by Reddit.

Not the same exact software, just the same way of representing comment threads.

Collapsing threads, for one thing.

It's like, one line of javascript different (the equivalent of if score < threshold: thread.collapse())

I don't care about the code. Collapsing is a significant change in UX.

EXACTLY!

No one seems to get this, not HN, not Quora, Digg (though that doesn't matter any longer...)

Its about increasing the information consumption rate and navigation rate. Collapsing is critical to navigating threads with thousands of comments, even tens of comments!

Any UX designer who does not see this, is, in my book, worthless.

Inspect the source for both sites. I have to admit, I felt a little nauseous when I first read HN's markup & styling.