I can speak first hand to this issue... A massive number of submissions to ratemystartup are spam. I evaluate each of these submissions by hand and it's not always easy to do.

On ratemystartup users have editorialized content. On HN I would suggest that link posts are only permitted to users with X amount of karma. I am personally more of a HN reader than a contributor but I would gladly sacrifice the ability to post links (unless the karma threshold is reached) in exchange for maintaining a great HN reading experience.