> Part of me wishes HN had a really strong moderator that slapped down nay-sayers.

Slapping down dissenting opinions leads to an echo-chamber. One mans sceptic is another's "nay-sayer", I think community moderation (via votes) is filtering high value comments rather well,so specific moderation is hardly required: egregious flaming excepted.

> If you're not contributing something valuable, comment deleted

Just because you disagree with a given opinion does not make it worthless.