HN distills some of the best ideas from its predecessors:

1) Scoring system for stories (Digg) and comments (Slashdot)

2) Community based story submission (Kuro5hin)

3) Infinite indentation (Usenet and Forums)

It's also interesting to note what they didn't include in the design: Subjects for comments

HN's approach is not without challenges

1) Following the conversation gets tougher when there are a lot of levels. Who said what?

2) By reordering the posts according to points the user gets disoriented.