people in this community are unusually prone to feeling that they're stupid if they do badly at something

I suspect this is a result of the tacit assumption that "if you're not smart enough, you don't belong at LW". If most members are anything like me, this combined with the fact that they're probably used to being "the smart one" makes it extremely intimidating to post anything, and extremely de-motivational if they make a mistake.

In the interests of spreading the idea that it's ok if other people are smarter than you, I'll say that I'm quite certainly one of the less intelligent members of this community.

I've noticed in many people (myself included) a definite tendency to overvalue intelligence relative to practice.

Practice and expertise tend to be domain-specific - Scott isn't any better at darts or chess after playing all that pool. Even learning things like metacognition tend not to apply outside of the specific domain you've learned it in. Intelligence is one of the only things that gives you a general problem solving/task completion ability.