Something I see quite often in both submissions and published fiction (including movies) is plots that hinge on people taking implausibly inefficient approaches to achieving a goal.
It's certainly true that individuals and organizations quite often behave inefficiently. That's not what I'm talking about. I'm talking about a character or (more often) an organization or government choosing a really truly ridiculously inefficient and roundabout means to a straightforward end, when there would appear to be much simpler/easier approaches readily to hand. Sure, that too can happen in real life (can you say building papier-mâché aircraft in East Anglia to fool the Nazis? talk about unlikely!), but sometimes real life is awfully implausible.
This comes up a lot in time-travel stories. Say organization A wants to get rid of the leader of organization B. In too many stories, the very first idea that organization A comes up with as a viable approach is to invent time travel and then go back in time and kill leader B in his or her infancy.
(I suppose it's a little silly of me to presuppose that time travel is possible and then object to particular uses of it on plausibility grounds. I guess some of my argument is just my gut feeling about how people are likely to behave, given a ground situation.)
Of course, if time travel already exists and is cheap, this isn't so implausible. But I've seen stories in which, as far as I could tell, a government sank huge amounts of resources into creating time travel (sometimes with no reason to expect that it's even possible) in order to take down an opponent. And it just seems to me that in almost every situation, there've gotta be not just simpler ways to go about getting rid of the enemy leader, but ways that the government in question is more likely to try first.
Similarly, some uses of clones often run up against this difficulty. If you don't use the handwavey genre convention that clones can somehow be "force-grown" to adulthood in a matter of minutes or days, then growing a clone to adulthood from scratch is almost always gonna be more time-consuming and resource-intensive than other approaches. There are plenty of ways around this problem; still, there are too many clone stories that don't take this kind of economic plausibility into account.
Speaking of economic plausibility, it's long been a staple notion in science fiction that in the future, we'll move people to other planets to solve the population problem here at home. I had been reading sf for over 20 years before a friend pointed out to me that, unless you have basically free energy, that's ridiculous. There are over six billion people on Earth, and if the current growth rate remains unchanged (which is unlikely, but that's a separate question), we can expect that number to double in 60 years. So even if we could move three billion people to another planet today, that would only buy us 60 years before we were back to current numbers.
And anyway, moving that many people doesn't make any sense. How many people can you fit on a spaceship at once? Say you somehow develop a really humongous spaceship that can hold the entire population of New York City, about 8 million people. Then you would need "only" about 125 such spaceships (or 125 trips) to carry a billion people. How much energy does that take? How long does it take? How do you keep people from breeding while this relocation program is going on? And by the way, how do you keep people from caring about being removed from their ancestral homelands, and fighting bloody wars to stop the relocation? Sure, you can advertise about how much better things are off-planet (cf Blade Runner), but that's gotta be mighty effective advertising to get half the planet's population to leave behind everything they know.
If you postulate free energy, then you can take as many trips as you want--but if you've got free energy, there might be better things you could do with it to reduce population pressures than try to move three billion people to another planet.
Another example is fighting a war on the surface of a planet; as we discussed in that old journal entry, there are certainly plenty of reasonable justifications for that, and it's arguably a genre convention anyway, but often the author doesn't bother to justify it (or, apparently, to even think about whether it makes any sense), and that tends to annoy me.
Many conspiracy-theory plots fall into the category I'm talking about, of course. Which is more plausible: that the protagonist is nuts, or that thousands of people are all playing their parts perfectly with the sole goal of keeping our hero from finding out one particular fact? Of course, this example shows the limits of my complaint, too; there are plenty of great conspiracy-theory plots, and in some cases the conspiracies even end up sounding kind of plausible. Or else they're trying to make some broader point about society (think The Prisoner) and aren't so concerned about mundane plausibility. Or else they're using elaborate plans as a genre convention (as in the Bond movies). All that is fine; still, there are plenty of stories in which the good guys or the bad guys engage in ridiculously roundabout and inefficient ways to achieve a goal, simply because it'll be more dramatic than doing it the simple way. Or (and I think this is really what bugs me) because it didn't occur to the author, after they came up with the cool complex approach, that there might be a simpler or more likely way to do it.
(The famous whip/sword/gun scene in Raiders of the Lost Ark is an example of messing with genre conventions and viewer expectations in this area, as is the climax of Wizards.)
I think a lot of authors run into this kind of thing from a different angle: how many times have you shown your magnificently tightly plotted opus to your first reader, only to have them say "Why don't they just arrest the bad guy in the first scene and eliminate the need for the rest of the story?"
I guess all I'm really saying is: next time you're about to have your bad guys launch their entire space armada to chase down your hero's singleship; next time you're about to have your hero go back to the dawn of time so that they can plant a fake fossil in just the right place to cause a bad-guy archeologist to turn a fraction of a degree at exactly the right moment to let the good guy cheat at poker; next time you're about to have a rich character who's on the verge of death start growing a clone, in realtime, from an embryo, in order to have a replacement heart eighteen years later; stop and think about whether you have some sort of justification for it (and whether there might be an easier and/or more likely way to achieve the goal).
I don't care whether the justification is internal to the characters and institutions involved, or whether it's a meta-justification involving genre conventions and authorial intent; I just want the author to have some idea of why this is approximately the easiest, simplest, most effective, least expensive, or otherwise in some way best approach to achieving the goal.