Elements of a Good Fictional Mechanism

For a technology or magic to be satisfying and believable in a work of fiction, it needs to be consistent, predictable, and limited in scope. I start by giving some examples of good but disappointing fiction, some good examples, then explaining what I meant by limited, predictable, and consistent.

In Harry Potter and the Deathly Hallows (Book 7), I couldn't help but feel a bit cheated. How secure was Gringotts, really? I just didn't feel convinced that most of the action in the book was planned, or even explained well. It just happened, and that was that.

In Star Trek, a lot of cool things could be done with transporters. But the Federation wasn't age-regressing their most valuable admirals and captains, then duplicating them with a double buffer transporter, and transporting the duplicates over subspace straight into battle zones--nor were any of their men traveling in time and dimensions to find out how to win the next fight or evade that idiotic treaty that prevented them from developing cloaking devices. Oh, and the Borg missed each of these technologies too. Why doesn't anyone occasionally say “Let's go have a beer and sing about the good old days when you knew that we really were who we think we are--and what and when and where we seemed to be. End program?” They don't even joke about these uncertainties (well, maybe they do a little). The theme song and end credits must scramble their memories or something.

On the other hand, the societal implications of telepathy or empathic abilities got scary in Babylon 5, Star Wars, Firefly, Buffy, and in Asimov's foundation series; in Harry Dresden's world, and in Dean Koontz's books they also had both dark and light sides. Things get messy and complicated. Perhaps I stopped liking Star Trek because rules and formulae were a bit trite or overly simplistic. But there was a time I liked Star Trek (TNG) despite the Prime Directive and magic reset button.

So what is it that makes a spell or gadget system believable? What makes it satisfying? Why is it anticlimactic if the dashing hero of the show wins the day by pulling out a heretofore unmentioned Divine Demon Gun and shooting the Hydra from Hell? It's because this gun came out of nowhere, and we feel like the writer didn't have time to actually find a logical way out for the hero. The victory is cheapened because it was stolen, not earned. It's only creative and clever when Dresden can't always use his blasting rod, or Hari Seldon's mathematics can't always predict the future accurately, or something else makes the path forward possible but not guaranteed.

So what elements make for a believable spell, gadget, techology, or magic? What makes the supernatural or the futuristic stuff more interesting, believable and satisfying? I have identified 3 or 4 categories.

Limited, or Scoped - This means that the machine or magic has limitations. A guy that breathes fire can't engulf a plane with his fiery breath. Limitations can come in many flavors: an expendable item; a slow or a delayed effect; technology that is unreliable; spells that half the population is immune to; a spell or gadget is expensive or has nasty side effects; and so forth. These limitations make it possible for the audience to know that not the heroes have hope, but victory isn't guaranteed--that even though the villains have power, they also have weaknesses. Not anyone can afford kryptonite; Luthor has to be close to superman to use it, and its effect is only temporary. Buffy isn't Hercules, and Hercules isn't Zeus; Luke can't defeat Darth Vader with a simple Jedi trick, nor can Vader destroy all the rebels without luring Luke to the dark side.
Consistent - This means that the power mechanism operates on the same principles as it did when it was introduced. The force doesn't operate differently for Luke than Obi-wan--their difference is in experience and mastery alone. Wolverine's adamantium is susceptible to magnetic forces, and this just isn't going to change unless something drastic happens (which it did). The One Ring always makes its wearer invisible and menacing (no matter who wears it), but it also gradually darkens his heart into selfishness and greed. These rules of consistency doesn't mean new twists can't be introduced (such as an antidote to mutant powers), but they have to be explained or foreshadowed first. Consistency means that the writer sticks with the natural consequences of his imaginary inventions, even when it's annoyingly inconvenient. The reader is not kept in the dark when all the characters in the book know what is going on.
Predictable - This means that thoughtful readers can understand the fictional mechanism well enough to predict whether something is possible or not, and what the likely side effects would be. For example, once you learn how Wolverine's claws work, you can reasonably predict that he can't cut down the Hoover Dam with them. You also know he is dangerously vulnerable to Magneto's powers. Because the readers understand how and why things are happening (at least partially), they can anticipate, predict, and guess at the future possibilities. This is where Harry Potter fails--we can never guess what spells the characters will pull out of their hats or what limitations they might have; magic is opaque to us. Good mechanisms allow the reader to say "cool--I wish I had seen that coming," because they know they could have or even should have. A reader will feel cheated if a completely random solution pops out of nowhere. This isn't to say that the entire world has to be explained well. In Lord of the Rings, we didn't know exactly how powerful the One Ring was, and we never did find out. But neither did the characters we were traveling with, so our limitation was also theirs--and it became a weakness: they were burdened by it, surprised by it, and afraid of it (rightly so).
Synthesizable - This is part of the predictable element, but it deserves some of its own discussion. There are logical, creative uses or combinations of various mechanisms that give rise to unexpected but undeniably logical choices and effects dealing with the fictional mechanism. For example, in one novel a protagonist uses an powerful air spell (which had been introduced earlier) to propel an elevator car up its shaft and crush a monster on top of the elevator. These are the kinds of uses that make the reader realize there is some real creativity: magic is used along with the laws of physics, or vice versa. Someone mixes technology with political influence. Military strategy changes because of a seemingly innocuous new spell.