it seems quite likely that we're under investing in sports/celebrity references-I'm quite certain that there are concepts that we discuss which would benefit from comparison to sports/celebrities.
But to grasp those concepts we'd need to actually learn a lot of mostly-useless, occasionally-relevant facts.
When an LW post says Gandalf should have brought Frodo to Rivendell as soon as he suspected he had the One Ring, the whole point of using that example is that readers understand it. If the post had to explain who Gandalf was, there would be no benefit in mentioning Gandalf at all. If the post instead talked about a pop culture reference that most readers wouldn't get, the post's value would be lower.
Yes, but at the cost of making it less accessible to the average person. Yes, given that there are very few people who will click with LW who don't have our community's cultural knowledge, we should bias (across all posts, comments) toward sf/fantasy/science/tech/whatever, but there are other factors in play, which mean that the current mix is too far away from the mainstream. Given that we enjoy our culture, we should expect that we overuse it, approve of it out of proportion to its worth, even given that our enjoyment of it is a reason to use it, approve of it.
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.