Supposedly a grad student somewhere did his or her thesis on Nora Roberts (I would love to link to it but can't find it). On surveying the voluminous work of Ms. Roberts said grad student noticed a number of common structural patterns: one of them was that the protagonist kisses the love interest for the first time after the same percentage of the book has elapsed. Learning this, you're left with the feeling that Roberts could just be doing a find-and-replace with character names and dropping in the odd paragraph of location description. ("In September Darcy smelled the air along the New England coast turning crisp..."-CTRL-H-"In October Jessica's boots crunched in the first snowfall around their little Rocky Mountain cabin...")
There are two reasons this post is not an attack on Nora Roberts. The first is that people are obviously getting something out of reading her novels, and if I could make what she does writing romance novels, I sure as hell wouldn't be sitting here writing blog entries for free. The second is that in my old age I'm getting picky about science fiction because it's more and more obviously doing the same thing. I'm not complaining that the genre has changed for the worse - it's that readers have access to more material, and more organized criticism of that material. This is why structural awareness of art, and structural approaches to creating it (whatever term you use for that) inevitably emerges in highly literate societies. (As a long aside: speaking of structural awareness in literature, have you noticed all the works like Pride and Prejudice and Zombies, or that Kerouac meets Lovecraft stuff, or techno-thriller meets space opera meets cyberpunk? You can't help but wonder what this means for genres or literature in general; there's a fine line between deliberate genre-cross-fertilization and parody, if it exists at all. The mid-90s to early aughts saw a burst of ill-remembered experimentation between metal and other forms of music, driven both by the stylistic dead end metal found itself in as well as a desire for broader recognition by music fans in general. Spot the difference.)
The bottom line: the more of a genre you're exposed to, the more obvious the underlying (identical) patterns and themes become, and more and more much of it seems like a find-and-replace exercise, even though the authors may not realize it themselves. If you're dubious, go here and click around for awhile (but come back and finish the post). It's a nice Wiki of science fiction concepts. Eldritch abominations, stellar empires, intelligent gerbils, all things which seem interesting and "cool" in novel settings - until you see multiple specimens of each concept isolated and under glass. Put them in a card deck and shuffle, connect these still frames with some fairly automatic narrative, voila! Science fiction story. In a way it doesn't even seem as clever as David Cope's program in the post I wrote before this one. The absent aliens don't always reveal themselves on page 183 as is claimed with Nora Roberts, but they still make an appearance in the same sort of pattern. There is a finite number of themes in any genre, including speculative fiction, and they get rearranged in different stories. Understandable; new ideas are hard to find, and even harder to read. After considering this, it might not seem so strange that a technical journal article with just one explicitly-presented novel idea is so dense. By analogy: throughout my twenties, I used to visit hospitals all over the country for my job, and I was amazed at how hard they were to navigate, and how you never get any better at it; you don't realize how similar are the floorplans of most buildings, and how much you rely on that, until you're in a large and unique building.
This post doesn't purport to be a revelation from a world-weary fiction consumer that there's no such thing as a new idea (that idea sure isn't new at least.) There are, but you have to dig; and in a way I'm sorry that I've finally stored up enough data that I can no longer avoid noticing the patterns. I imagine this will be happening to younger and younger people as time goes on, as media becomes more prevalent and available and sortable.
Genre fiction will always be guilty of trope-abuse to some degree; otherwise it wouldn't be genre fiction. But some are worse than others, and fantasy has always been one of the guiltiest. The evolution of fiction is about turning more and more of the traditional elements of literature into variables instead of constants, and speculative fiction finally brought even setting into play. That's why fantasy always seemed like such a wasteful genre to me - a speculative fiction author is free to build whatever world he or she wants to, and the fantasy author takes that opportunity and puts a character into a re-named early medieval Europe, complete with the looming columns of a recently fallen empire. Yes, I know, people like sword-fighting, so it sells; fine for them. Not everybody wants to spend their finite minutes skimming repeatedly through the same duel.
This might be why in our dotage, sf fans eventually move to things like Borges or Barth (although Wolfe and Delany also force some number-crunching). What do you really get out of a find-and-replace-the-names kind of piece except temporary amusement? It's possible and preferable to learn something about the real world through fiction and even to exercise your critical thinking and problem-solving muscles.