My guess is that the most important factor in the success of HN is Paul Graham's essays, and the only reason https://lesswrong.com got any traction is Eliezer Yudkowsky's essays.
For me it is the moderators and all the algorithms to keep the Troll’s away (that you first notice have a good effect after a long time usage, if you are looking for them).
Examples: Up/down-vote, flag, push down posts with a lot of discussions without new up-votes, republish posts that gets votes late...
