You don't need to overcomplicate things. Simply put Nerds are people who don't fit in with the societal norms. It doesn't even have to involve fandom, although that can go along with it. If don't fit in, your a nerd.
Similarities between fantasy fandom, and sports fandom can be made, but by and largely come down to intellect. Not everyone can get their heads around a lot of science fiction and fantasy concepts, and even if they can none of it is real. On the other hand "Man that can jump really high" is something anyone can understand, and an accomplishment that can be appreciated in of itself. Thus anyone can get behind something that is real, verifyable, and involves an activity that they could participate in themselves with an actual, tangible result even if they are not as successful at it.
Everyone can pretty much understand the fundemental idea of kicking a ball into a certain area where other guys try and stop you. Everyone can understand the same thing with putting a ball through a hoop. It's very simple, and very tangible. Not everyone can understand or appreciate say a paper and pencil RPG where all the action happens in your imagination and people can spend hours talking about fantastic concepts, without producing anything tangible, and somehow be entertained.
You'll notice that over the years a lot of things that WERE Nerd domain have been increasingly dumbed down an simplified which is why the mainstream have managed to get involved in these games. A point being made frequently that for the original group of fans these dumbed down games have basically ruined gaming. It's sort of like where if you ran sporting events "Harrison Bergeron" style where drugs were administered to all of the athletes so nobody would have an unfair advantage. "Man who can jump really high" becomes "Man who can jump like everyone else", kind of ruins the whole thing.
You can see this issue discussed here on "The Escapist" in various forms with a divide between "real gamers" and "casual" gamers which is something I'm not going to rehash here since this post is long enough and I have no idea who is bothering to read it as it is.
When it comes to comic books, I think they represent a rather unusual medium. To be honest I think the problem with comics is that due to societal panic reflexs, similar to the current anti-violent video game crusades, they wound up being regulated to the point where it was impossible for them to grow up with the people reading them. They were very simple stories, whih took place in idealized worlds, and could never progress beyond that. Thus it took a rare kind of person to remain interested in them beyond a few years in childhood. Though to be honest comic book fans and collectors were never really scorned *JUST* for that, since everyone read them at some point, and people can understand both nostolgia, and the motivation to collect something. The current success of the comic book-inspired movies is because they are demonstrating to a mainstream that had by and large moved on that comics had broken free of the previous conventions and have been able to grow up (somewhat) while still remaining very simple.
"Star Wars" is also an interesting case because it's not really science fiction, it's "Space Fantasy" whether anyone wants to admit it or not. The entire thing is pretty much a fairy tale set in space (and like many fairy tales in their original forms, was very dark in it's initial conception, but that's another whole discussion). Even the "A long, long, time ago, in a galaxy far, far, away" is meant to invoke the image of the classic childhood tale.
Star Wars pretty much throws flashy, easy-to understand stuff out there, without bothering to explain how any of it works or makes sense. Sure there are nerds who took it to a whole new level and demanded explanations (or created them), but to the typical viewer it's pretty much "ooooh, pretty... look at all the lights and FX. Wow is that a metal bikini?".
Compare this to say reading a Science Fiction novel. First you need to be able to read at a reasonably high level (and literacy has been a problem in the US for decades sadly). Then if you are reading actual science fiction, nothing just happens purely because it's cool. If you read things like Pier's Anthony's "Bio Of A Space Tyrant" series for example, above and beyond all of the gunfights, ship battles, and (frequently kinky) sex, are explanations on how everything works and why things are happening. Why ships are shaped like spheres, how and why battles are fought largely by "Carrier Ships" using AI controlled drones, the motivations and techniques of space piracy, the subculture of migrant labourers in the far future. All of these things are parts of the story, you omit that kind of stuff and your typically left with a bunch of wierd looking space ships and mindless violence and sex with nothing to explain why anything works the way it does. This is by and large why science fiction movies failed on such an epic level before "Star Wars" which pretty much was developed to use the trappings of sci-fi with an intentionally simplistic storyline... and it worked. Today the remake of "Battlestar Galactica" is about as deep as something like this gets for the mainstream, and it's like a kiddy pool compared to serious science fiction.
I think perhaps the best example of the problem can sort of be illustrated in the movie "Battlefield Earth". I won't say that the book on which it's based is wonderful, but it's far from the worst thing out there. When you hear people in the mainstream talking about that movie you hear people talk about how "dumb" it is that a bunch of savages could learn to fly planes using simulators in what seemed to be a ridiculously short period of time, or how a race that had obliterated human civilization inside of 4 minutes could be so totally stomped (even in a smaller scale battle) by refurbished 20th century weapons. Of course if you read the book you'd know that the "uprising" took quite a while to get going, and also the key element was that the aliens were arrogant and managed to win due to having total surprise. Their weapons tech as far a face to face confrontation wasn't any better, they had just developed in some other areas (a point not unique to this story, I've mentioned it in other posts discussiong alien invasions and the like). The bottom line was that the aliens themselves were surprised the same way (as I remember there was an analogy), and never having actually engaged the human military due to it being surprised they had no real respect for the fact that in atmosphere that fighter jet is still going multiple mach speeds (just like your fighters) and your ships are not going to ignore getting hit by a couple of laser guided missles packing explosive warheads. Sure you might have a fancy hand held energy cannon that can put a hole in something, but if you get hit by bullets from an assault rifle your just as dead. Of course watching that movie, you'd never pick up on any of this, and it was all glossed over. If you knew the book somewhat to begin with you could get where it was coming from, but due to it not being spelled out (and under a time limit as a movie) the thing suffered to mainstream audiences. Only with very long series like "Stargate" do you see much discussion of concepts, and that's something that is gradually worked in over YEARS if a series survives at a very basic level.
