Endless walls, endless trains, endless clouds.
Uniformity, monotony, apathy. They make for drearier reading than a blank page (at least a blank page is hope’s canvas). Hence Kurt Vonnegut’s counsel to aspiring authors:
Make [your] characters want something right away even if it’s only a glass of water. Characters paralyzed by the meaninglessness of modern life still have to drink water from time to time.
(From his interview with The Paris Review in 1977.).
Needs must when nature drives.
Wants give the reader a foothold in the story: What do you think of a man dying of thirst because he cannot reach the glass on his bedside table? Or of a political activist refusing a glass of water as part of her protest fast until she is force-fed?
Opinion is hardly dispassionate. A meagre glass of water will elicit something in even the most desensitised reader (pity, bile, fever), and the emotional investment in another’s hardship—be it fictional—amounts to attention.
Generating hardship is the storyteller’s prerogative and duty, generating it any which way, usually by an idiosyncratic magic opaque to others. But before the twirl of the wand happens, the elements of the craft are strategically employed: the opening paragraph hooks the reader, story parts flow into one another, the final punch is delivered with due panache. Ultimately learnable, practicable, and discernible, these elements are the ideal backdrop against which to measure the effect of the wand’s hocus-pocus.
Metaphor is a decent measure.
For example, the fantastic horror stories in The Dark Eidolon by Clark Ashton Smith are dark chocolate filled with hazelnuts bits—consistently rich and bitter and filled with interesting words that texturise. On the other hand, the experimental, wordplay stories of The Cartesian Sonata by William H. Gass are puzzlers in half-dissembled states that at first push or prod make little sense—the story elements are disguised by unusually jagged word-configurations.
Your precise metaphor doesn’t matter so long as it captures the resistance factor that breaks textual uniformity and that holds your attention (e.g. hazelnuts of Ashton; configurational puzzlers of Gass). In fact, your metaphor is necessarily unique, because your entry into a text comes from a different point and from a different angle to everyone else’s. Barthes has his own way of thinking about it:
If you hammer a nail into a piece of wood, the wood has a different resistance according to the place you attack it: we say that wood is not isotropic. Neither is the text: the edges, the seam, are unpredictable.
—Roland Barthes in The Pleasure of the Text, translated by Richard Miller.
I may hit knots in the wood often (hazelnuts, puzzles). You may hit none (and find the text as pointless as counting snowflakes).
Vonnegut speaks of want in fiction—the predictable want that anchors the character (and reader) by resisting the chaotic ocean of life—but Barthes speaks more generally of resistance in any text, as the property that generates interesting edges and seams.
Indeed, there’s no reason to restrict the metaphor that measures magic (or resistance) to fictional texts. For example, from my reading list:
The essays in Modern Man in Search of a Soul by C. G. Jung are a light show fighting a persistently seeping black canvas—trying for wholesome as contrasted to Freud’s dark underside.
Dancer by Colum McCann is a dizzying gymnastics routine with many colourful acts—switching between different writing styles and characters to give a fuller picture of Rudolf Nureyev’s life, never letting you ask too many questions.
Phaedrus by Plato is a clever jester with multiple masks where you’re alway peering to see if this is the final one. Hallucinations by Oliver Sacks is a magic lantern in a cabinet of mind curiosities where science and awe mingle. Lorca’s poetry is the moon and the mirror and you can’t know which is which… and so on and so on.
Acknowledging the metaphor, acknowledges the magic. It’s the reader saying: I know I’m spellbound, I know how and why, and that’s alright, because that’s precisely what I asked for.
A Quiver Of Quotes 
I want there to be nuts in the chocolate – but I won’t struggle with them if the writer is only seeming to be showing off how much more clever he is than all the others.
Often in class I would try to teach students the essence of simplicity and then license them to become complicated as they grew.
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What I think are nuts might be stones to someone else, or just plain invisible air bubbles.
I’m not sure how many authors I’ve read where I felt they were showing off or being condescending in a way which I didn’t approve of (can’t think of any at the moment probably because I stopped reading them ASAP and promptly forgot about them).
But yes, simplicity first, complexity later and only as organic growth of the author.
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I’m still slightly annoyed that we can’t sit and have coffee and talk for an hour.
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