It is probably the case that structuralist, or formalist, approaches to SFF predominate today, and there are good reasons why they do. The urge to categorise, obviously, has its cognate in science itself—the Linnaean, taxonomic impulse—but that’s a fact that makes it more, not less, questionable: in the sense of imposing a sort of necessity of deconstructing the assumptions underlying the impulse to categorise. What a structuralist account of SFF does is not so much bring ‘order’ to the flux of this body of text, as to prioritize the notion that categorizing itself mediates textual desire.
And of course, there is a sense in which it does: there is pleasure in spotting similarities and parallels between things, and grander totalizing pleasure in disposing of a large body of diverse individual texts into a small number of pigeonholes. The pleasure, to put it bluntly, has to do with control; and when it is applied to the world (as Linnaeus did) there is some point to it, for the world, for most human history, has been hostile and even dangerous. But when it is applied to SFF it misses the crucial thing that draws us to these texts in the first place: not the illusion of control (power), but the sense of transport.
Broadly speaking, this is what is distinctive about the appeal of SFF texts to fans of SFF. The technical vocabulary of criticism, by talking about ‘novums’ and ‘estrangement’ and ‘structural fabulation’, although they are talking about this thing, don’t sound as if they are, which may be a distraction. Closer to the money-shot is the descriptor ‘Fantasy’ itself: a word which has a spread of meanings, not necessarily negative or merely escapist in connotation, for the world of psychoanalysis. Why ‘fantasy’, then? Or, perhaps it would be better to say: what is behind the desire for fantasy?
Again, speaking very broadly readers of Fantasy pick up their favourite books because those books give them something missing from the world as it actually is (and missing, usually, from artistic representations of the way the world actually is; or ‘realism’ as it is sometimes called). We might call this thing ‘enchantment’, a sense of magic. Readers of SF are in search of something similar in their preferred genre: a newness that the actual world lacks—hence, of course, novum; except that it is too easy to imagine that this newness inheres in one or other prop or physical item (a time machine, a ray gun, a spaceship). But this is to reduce SF to gadgets; and the problem with that is that the world itself has no lack of gadgets—is, indeed, rather over-supplied with gadgets. Better to talk in terms of ‘sense of wonder’, provided we realize that this in practice is a slightly less rebarbatively awe-inspiring quality than the eighteenth-century ‘Sublime’ (sometimes used as a synonym). It might, in fact, be best to think in terms of ‘cool’ if that didn’t carry with it the odour of imprecision.
Fantasy carries us away. We want it to—that is why we go to it in the first place. As to why we get such pleasure in being carried away (get such pleasure, not to put a finer point on it, by focusing on what the world is missing, on its lack) ... this is a large question and I’ll come to it in a moment. But to begin with it’s worth dwelling momentarily on this trope of ‘carrying away.’
The difference between a metaphor and a simile is a question of semantic nicety that some people find hard to articulate. This is perhaps because there isn’t really a difference; the two words are used more-or-less interchangeably in many contexts. But I like to insist upon a difference for all that: simile, as the word suggests, is a way of talking about something by comparing it to something that is similar: ‘Achilles is courageous, like a lion’ focuses our attention on the point of likeness. The word metaphor, as rhetoricians remind us, means a carrying over, a passage of meaning from one thing to another thing. This might sound like hairsplitting, but there is a difference here, and it seems to me one that opens a chasm of signification that speaks directly to the desire at the heart of SFF. ‘Achilles is a lion’ metaphorically carries across from one thing to a completely different thing. Because, crucially, Achilles is not a lion—there are a wealth of ways in which Achilles and a lion are different. To say ‘Achilles is metaphorically a lion’ is in one part to bring out a point of simile (in this one respect—his courage—Achilles is a lion) but it is always, inevitably, to do much more: it is to generate (in Samuel Delany’s words) an imaginative surplus, a spectral hybrid of beast-human.
This imaginative surplus is what carries us away; and metaphor is its vehicle. That is partly what I mean when I talk about SFF as being in crucial ways a metaphorical literature: one that seeks to represent the world without reproducing it.
‘Desire’ then is, I’m suggesting, at the heart of SFF’s appeal; and I’m saying something else—I saying that, whilst desire is also at the heart of the structuralist, systematizing urge, it is a desire radically opposed to the desire we call Fantasy. Fantasy, in a healthful, ludic, rejuvenating way, is precisely about escaping the grid. It is about the imaginative and affective surplus, the overspill. Indeed, I’m tempted to say, because this is the case, the desire of Fantasy (let’s qualify it a little: of the best Fantasy—and without wanting to sound circular, I’d suggest that this is in fact by way of identifying what it is about those texts that makes them the best) comprehends the excessive nature of desire itself.
It tells us nothing about the reason so many people fall in love with (the phrase is not hyperbolic) The Lord of the Rings, to say that it is a portal-quest fantasy. That is indeed a feature of the text, and one it shares with many other texts; but most of these others texts are not enchanting (we do not fall in love with them) in the way we do with Tolkien. Actually The Lord of the Rings is a book precisely about desire, and what is so canny in its delineation of the operation of that desire is the way it dramatizes it as simultaneously transporting and isolating; it excavates, we might say, our instinctive understanding that desire is captivating in a wonderful as well as an enslaving sense. It’s a striking thing, in this respect, that nobody doubts the intense desirability of the ring at the heart of the narrative, even though (in Tolkien’s rendering) it is never made explicit what it is the ring actually does. It has something to do with power, we're told; and the person who has the ring will be able to wield power—tyrannically—although at the same time the various people who have the ring in the book (Gollum, Frodo, Sam) seem to derive no social, or practical empowerment. Indeed, on the contrary: the efficacy of the artefact seems pointedly antisocial: it can make them disappear, it can remove them completely from the social body.
In the first film of Jackson’s trilogy, fatally, even bathetically, there is a moment in the prelude sequence where Sauron is shown wielding the ring: sweeping his arm on the battlefield and sending scores of warriors flying into the air. But this is a rare lapse of representational sophistication in a film-trilogy otherwise, I’d say, sensitive to the point of the text—subsequently Jackson abandons such literal-minded idiocy, and is much better about finding visual analogues for the ring’s appeal. Because this is the whole point. The ring signifies not some active mcguffin (to go back to the phraseology I was employing earlier: it is not a gadget). Rather the ring construes desire itself, and in doing so makes manifests its intense, destructive desirability, precisely as absence. It is something not there: a little hollow, a badge of literal invisibility, something associated with the dark in subterranean caverns or the inaccessibility of riverbeds. The ring is lack, and Tolkien’s brilliance is in understanding that lack is the currency of desire. Actually, and to digress momentarily, I’m not sure this is what Tolkien thought he was doing; I think he thought of his ring in terms of lack because he meant the ring to symbolize evil, and for his Boethian/Acquinian theological perspective on the world evil is absence: the world itself, as God, is necessarily good except insofar as it has been eroded or perverted by evil. But that doesn’t alter what I’m saying, I think. There are reasons why The Lord of the Rings has had the global impact it has, that its myriad imitators (which have all, like true structuralists, scrupulously copied the form of the portal-quest narrative) have not. LotR construes desire (readerly desire) because it understands desire.
Adam Phillips, in Side Effects (2006) has some interesting things to say about masturbation which, strange as it might seem, are relevant here. (Shatner, in that celebrated Saturday Night skit, touched a nerve—which is why fandom still loves that sketch—bellowing at the fan-geek wearing the ‘I Grok Spock’ T-shirt: ‘you! Have you ever had a girlfriend?’). Philips starts by quoting Leo Bersani:
Bersani once said in an interview that the reason most people feel guilty about masturbation is because they fear that masturbation is the truth about sex; that the truth about sex is that we would rather do it on out own, or that, indeed, we are doing it on our own even when we seem to all intents and purposes to be doing it with other people. The desire that apparently leads us towards other people can lead us away from them. Or we might feel that what we call desire is evoked by details, by signs, by gestures; that we fall for a smile or a tone of voice or a way of walking or a lifestyle, and not exactly for what we have learned to call a whole person; and that this evocation, this stirring of desire, releases us rather more into our own deliriums of fear and longing than into realistic apprehension of the supposed object of desire. There is nothing at once more isolating and oceanic than falling for someone. Lacan formulated the ‘objet petit a’ to show us that the promise of satisfaction always reminds us of a lack … and that this lack, disclosed by our longings, sends a depth charge into our histories. 
It would be almost fatuous to note that the ring, in LotR, is an objet petit a—fatuous, really only because it is so extraordinarily obvious that this is what the ring is. But it’s another phrase from that little passage that leaps out at me in the context of understanding the desire behind SFF: ‘there is nothing at once more isolating and oceanic than falling for someone.’ That’s right, I think, as an account of what it is like to fall in love with someone. More than that, though, those two words, ‘isolating and oceanic’, seem to me wonderfully apt as a way of approaching how the best fantasy wins us.
The core of Tolkien’s book, then, is its apprehension—through its concrete realization, its worldbuilding and backhistory and characterization and so on—of the radical undesirability of desire; or the desirability of the undesirable. The point is that the phrasal superposition of desire and undesired only looks like a paradox. Actually it is an articulation of something much more significant. Philips again:
Anna Freud once said that in your dreams you can have your eggs cooked any way you want them, but you can’t eat them. The implication is clear: magic is satisfying but reality is nourishing … Indeed, we could reverse Anna Freud’s formulation and say that when it comes to sexuality it is the fact that you can’t eat the eggs that makes them so satisfying. The fact that, as Freud remarked, desire is always in excess of the object’s capacity to satisfy it is the point not the problem; it is the tribute the solitary desiring individual pays to reality. This is a problem only if you are a literalist rather than the ironist of your own desire. It’s not that reality is disappointing, it’s that desire is excessive. It’s not that we lack things, it’s just that there are things we want. 
In this passage I’m tempted to replace ‘dreams’ with ‘Fantasies’, and to extend the observation to those novelistic excrescences of fantasy life booksellers label under that term. And I’m tempted to suggest that ‘sex’, here, connects with the fundamentally libidinous energies that flow through our love for these narratives.