I would equate intelligence with basically how good one is at learning in general, without giving priority to some fields. I think open mindedness and curiosity are crucial traits for that.

Maybe I was above average in, say, my high school graduating class, but I doubt that is true of the Less Wrong community. People wouldn't be here if they lacked that degree of open-mindedness and curiosity.

They don't get to learn at their own pace for their own intrinsic motivations. This commonly alienates people from the subject. Causes like these are cultural.

Would you like to comment on how non-Western cultures view math differently? Or offer a suggestion as to why I was the only white girl in my high school calculus and vectors class? (I like math a lot...it's just that most people who like math like it because they're good at it, so the only people who want to talk to me about math and how awesome/fascinating it is are usually massively better at it than I am, which may be why I perceive myself as not being good at it.)

I do have stubbornness, which can be an advantage to learning new things (I spent 8 years teaching myself to sing, and went from complete tone-deafness to composing my own piano and vocal pieces and performing moderately difficult solos.) I am also stubbornly loyal to prior commitments, which basically means that once I start doing something I never stop...after awhile this limits my ability to start new things. (I can't teach myself quantum mechanics while I'm working 2 jobs, singing in a church choir, and going to school full-time.)

And for quantum physics in particular the situation is pretty terrible (if you want to learn it in depth; there's OK popular science books for a lower level of detail).

Agreed! I ran into exactly this problem; I've read enough pop science books that I no longer learn anything new from them, but when I took a textbook out of the university library, I took one look at the first page and was lost. Eliezer's intro to quantum mechanics would probably help, if I made the commitment to go through it entirely and practice all the math, but again, not something I can do very easily on my breaks at work.