(Slices of American cheese don't make good night vision goggles)Every piece of fiction requires that we indulge in a bit of fantasy. We have to believe that magic exists if we're to enjoy the Harry Potter series, just as we need to ignore the laws of physics and accept faster than light travel if we want to have fun with George Lucas and Star Wars. These leaps of faith are benign and necessary bits in losing ourselves in the larger world.
However, authors don't always make these leaps easy. There needs to be some aura of believability in the story or we'll just roll our eyes and move on to something else. I can personally let a lot of stuff go, but the one area I can't let go in is with regards to human reaction and common sense.
I admit that I have a tendency to over think things at times - my wife says it's one of my best qualities - but I have a keen grasp of how the real world works and what's plausible. That helps keep me grounded, and it also affects the way I write. One of the reasons I couldn't get into the movie Avatar at all was the sheer idiocy of it all - if humanity had the technology to cross the stars and get to Pandora, and if they had knowledge of the location of the mineral they were there to get, why in the world would they risk a close quarter combat action against a foe who can only do you harm if you close with them? Why wouldn't the mercenaries simply bombard the planet from orbit and take the mineral at their leisure?
Writers have a lazy tendency sometimes to just make outlandish leaps in our stories so that the story will work out the way we want. The down and out family will win the lotto just before their house is foreclosed on, or our super-villain who was smart enough to determine the heroes latest plans will forget to activate the one camera at the lone spot our hero slips through to get inside the secret compound. These things reduce our stories to parody, and they lower the intelligence of our work.
Don't make it easy as a writer. You have to assume your audience is as smart as you are. Don't exclude that subtle plot point that won't be important for another 50 pages - leave it in. Have your antagonist remember to load a full clip into his weapon so your hero has to actually figure out a stealthy approach. The average reader is smart - that's why they're reading - so reward their intelligence and allow them to believe your world. If you can't grant them that simple courtesy, why should they ever read your work?