The failure mode here doesn't come out of Less Wrong being ontologically nerdy but the potential for it to become teleologically nerdy. To clarify, the topics of analysis that we see here are going to be seen as nerdy by most of the mainstream; short of changing the culture there's no way around that. But assuming that rationality and nerdiness are causally entangled with each other, that epistemic rationality implies nerdiness in other domains or vice versa, is not only wrong but dangerously so.
The community seems to be fairly adept at avoiding the general class of failures that this belongs to; it's managed to stay mostly clear of them in the political domain, for example. That's good. But the wider nerd culture is extremely bad about confusing its cultural preferences with its notion of rationality, as the appropriate fandoms' attitudes toward Justin Bieber listeners, Stephenie Meyer readers, and Wii Fit players should show. At times it can shade into something that almost resembles a religion, as in Neal Stephenson's In The Beginning... Was The Command Line. With this in mind, and bearing in mind that this community is by any measure full of nerds, it ought to be guarded against specifically.
We might end up attracting more and more rational people if we branch our pop-cultural examples out of fandom and into more mainstream domains, though that's only as effective as demographics and cultural norms will allow. But what we lose (or gain) from not doing so is absolutely insignificant compared to the potential bias we could introduce by entangling the myths of our tribe with the laws of the universe.
But the wider nerd culture is extremely bad about confusing its cultural preferences with its notion of rationality, as the appropriate fandoms' attitudes toward [...] Stephenie Meyer readers
At least we don't have that specific problem.
Less Wrong is as a community extremely nerdy. That's true for almost any definition of "nerd" that captures anyone's intuition for the word. However, to a large extent, many aspects of nerdiness are not connected to rationality at all, even though nerdiness may be associated with more rationality in some limited aspects. For example, fantasy literature is probably not in any deep way connected to either intelligent or rational thinking except for historical reasons.
Yet LW is full of references to science fiction, fantasy literature, anime and D&D. In one recent example, a post started with an only marginally connected tidbit from Heinlein. Moreover, substantial subthreads have arisen bashing aspects of other subcultures. For example, see this subthread where multiple users discuss how spectator sports are "banal" and "pointless". I suspect that this attitude may be turning away not only non-nerds but even the somewhat nerdy who enjoy watching sports, and see it has harmless tribalist fun, not very different than friends arguing over whether Star Wars or Star Trek is superior which has about the same degree of actual value here.
There's a related issue which is a serious point about rationality and human cognition: Our hobbies are to a large extent functions of our specific upbringings and surrounding culture. That some people prefer one form of fantastic escapism involving imaginary spaceships isn't at some level very different than the escapism of watching some people throw and catch objects. Looking down on other people because of these sorts of preferences is unhelpful tribalism. It might feel good, and it might be fun, but it isn't helpful.