I just read the survey from May 2009 than jimrandomh posted a link to. I found it extremely interesting. I would guess, from the postings I've read, that the demographics haven't changed drastically since that survey was taken.
Re: IQ's- I remember from one of my psychology classes that the 98th percentile is an IQ of 130 and the 99th is 150. I find it quite believable that the average IQ on this board is in the 140's. I know my IQ is in the 98th or 99th percentile, based on how I did on other standardized tests and I feel like the dumbest person on the board when I read many of the posts.
I recommend being selective about recommending this board. If someone fits into the demographics and seems like he would like discussing the common topics, go ahead- but it is definitely not for most people.
I've been lurking on LW since shortly after it started, and on OB for about six months before that. In that time, I've told four or five people about it. I would make a terrible evangelist.
I'm curious as to whether other people have the same problem. I'd like to tell lots of people about LW, but I don't think they're ready for it. If they read a statement like "purchase utilons and warm fuzzies separately" their eyes would glaze over, and they'd walk away thinking LW was some sort of crackpot site.
I have found certain posts and topics to be fairly good hooks for getting people interested. An Alien God is a good suggested read for people with an interest in evolutionary theory and the Cthulhu mythos (surprisingly high crossover in my experience). HP:MoR is also a pretty popular hook. The site itself isn't really optimised for word-of-mouth, though, and not everyone likes child wizards and blasphemous horrors.
How many people have you introduced to LW? Who were they, how do you do it and what was their reaction? How could we do it better?