So lots of people gather on HN because it is so good and soon enough someone realizes that posting links on HN is a good way to gather page hits. It's downhill from there. People begin to post not only about others startups they find interesting, but about their own ones too for fun and profit. Intentionally or unintentionally astro turfing. Then genuinely intersting links also gets accused of being link bait or of promoting some business.

Not before long you'll find shady ads on various forums selling links on HN: "tons of karma, 2yr acnt, $20$/ppost" Not a single site built on user submitted content have been able to withstand that plague.