By way of disclaimer, I'm neurotypical, bar a childhood ADHD diagnosis that I'm quite skeptical about.
In any case, I agree that it's pretty common for Internet communities to place an unusual and perhaps undue emphasis on neurodiversity, possibly because of the subculture's generally high Openness (although other reasonable hypotheses exist); I did time on TV Tropes for a while, for example, and in its later years it got to be about as dismal a hive of self-congratulation as I could describe. I've heard the superpower analogy too, and I don't find it a whole lot more convincing than you seem to.
I don't think this is a signaling phenomenon, though, at least not primarily: a diagnosis carries certain sick-role implications that might be useful from a signaling perspective, and it might also further a desire to be seen as unique in some subcultures, but neither one seems to match the actual behavior involved. I think the explanation's simpler: a lot of bits have been spilled on how the Internet fosters self-reinforcing identity groups, and I think it'd be naive to suppose that this tendency doesn't extend to identity categories based on some kernel of neurodiversity. The broader culture's also gotten a lot more welcoming of small identity groups in recent decades; this might be what you're getting at with your mention of political correctness, although I'd definitely hesitate to use that term. And none of this is fundamentally a bad thing. It can lead to bad things, if it fosters fixation of destructive traits that might otherwise wash themselves out of the memetic ecosystem, but I don't think that's worth complaining about here; LW's always been fairly welcoming of self-improvement as an objective.
The rest comes down to a nature-versus-nurture debate, and this isn't really the place; but this thread, I'd say, is fairly harmless. It's not hugely useful from a theoretical perspective, being self-sampled, and in any case I don't really expect to find any unusual neurological correlates of rationality, but if an anecdote or two ends up being interesting or inspirational from a cognitive-science point of view I'd count it as a net positive.
I see what you're saying, and I guess I perhaps should have considered making a discussion post on the topic rather than throwing my rant in this thread.
Partially to help reduce the typical mind fallacy and partially because I'm curious, I'm thinking about writing either an essay or a book with plenty of examples about ways by which human minds differ. From commonly known and ordinary, like differences in sexual orientation, to the rare and seemingly impossible, like motion blindness.
To do this, I need to start collecting examples. In what ways does your mind differ from what you think is the norm for most people?
I'm particularly interested in differences - small or large - that you didn't realize for a long time, automatically assuming that everyone was like you in that regard. It can even be something as trivial as always having conceptualized the passing of years as a visual timeline, and then finding out that not everyone does so. I'm also interested in links to blog posts where people talk about their own mental peculiarities, even if you didn't write them yourself. Also books and academic articles that you might think could be relevant.
Some of the content that I'm thinking about including are cultural differences in various things as recounted in the WEIRD article, differences in sexual and romantic orientation (such as mono/poly), differences in the ability to recover from setbacks, extroversion vs. introversion in terms of gaining/losing energy from social activity, differences in visualization ability, various cognitive differences ranging from autism to synesthesia to an inability to hear music in particular, differences in moral intuitions, differences in the way people think (visual vs. verbal vs. conceptual vs. something that I'm not aware of yet), differences in thinking styles (social/rational, reflectivity vs. impulsiveness) and various odd brain damage cases.
If you find this project interesting, consider spreading the link to this post or resharing my Google Plus update about it. Also, if you don't want to reply in public, feel free to send me a private message.