I think the reason that I like to check it a lot is to see what intelligent people think about the things that I say. So the first important component, in my mind, is that the majority of the members are smart. Secondly, by allowing people to reply to replies and assign karma points (admittedly not a novel idea), smart people can reinforce the parts of my replies that they find smart and ignore the parts that don't interest them.

I think the inability to vote topics down avoids some of the problems that Reddit has encountered, where people camp out and vote submissions down in order to make their own appear higher. I think a more mature (on average) user base helps to accomplish that as well.

Finally, I think that most people must have heard of YC news through either pg's site or through YCombinator itself. Anyone interested in those two things are cool in my book, and should produce good discussions.