Personally I feel quite strongly that 'niceness' is way too vague a concept to in any way promote, no matter the social context.

I'd like to talk instead about the value of comments that are specific, positive (+ hopefully warm, without gushing), and cooperative. In short, creating a norm of definite, positive, 'working together to work out what's true' attitudes. I think it is fine to make comments that only express approval, as long as it is approval of a specific behaviour / characteristics and not blanket 'good job'. These kinds of specific comments help people evaluate themselves and encourage them to continue doing what works.

LessWrong is not a debate club -- we're trying to approach the truth, not merely win the argument. That means that things which keep us working together on that are a net win, providing they do not obscure the truth.