They were pretty clear about it - they screwed up by underestimating the scale of the problems. It's better to admit your mistake quickly and try to fix things.
Instead of building a Web of Trust, a better solution might be to find efficient ways of clustering people.
Look how civil and insightful this discussion is. Why? Because people have different "quality", even if it's not politically correct to talk about it, we all know it intuitively.
Imagine a forum, where all the HN-level people are gradually clustered together, all the rednecks with conspiracy theories, all the leftists dreaming of communism, all form their own independent echo-chambers.
Yes, echo-chambers are bad, but let's face it, you won't be able to change most people's opinions anyway. Don't agree with me? Go on reddit, try saying something "controversial", like "men can't get pregnant", see how many people you would be able to convince :)
:)