Honest question. We know Google [1] (Bing and other s.e.) is fighting an endless war against link farms. AFAIK HN doesn't use rel="nofollow" [2] (EDIT: not entirely true, see comments below) so its link juice may flow to any website that is submitted. However, HN and its linked sites don't seem to be negatively affected nor penalized. What makes HN different from what Google consider a link farm?[1] https://support.google.com/webmasters/answer/66356
[2] https://support.google.com/webmasters/answer/96569