It's interesting to think about where individual fans of science fiction and fantasy draw their respective lines in the sand regarding the Suspension of Disbelief. Star Trek fans, for instance, accept warp speed, matter transporters; telepathy and the occasional alien with god-like powers without batting an eye. Have somebody steal Spock's brain, though, and we throw a collective fit. (And well we should--that was arguably the original series' worst episode.) That simply goes a step too far and crosses the Suspension of Disbelief line.
But then you'll occasional find a fan of the show who may criticize "Spock's Brain" for bad writing, but be perfectly okay with an alien race having that level of technology.
There's always things that can strike us as "wrong" even when they are no more unlikely or unrealistic than other elements within a particular story. If the stories set within a particular fictional universe are good enough, then that universe eventually establishes its own rules (not all of the deliberately created) regarding what should or should not be allowed. Toss in something that doesn't fit within those rules and it just doesn't feel right.
But because these rules aren't always explicitly laid out, individual readers can sometimes disagree about whether something does feel wrong.
I just read the novella "The Galaxy Raiders," by William P. McGivern, which was first published in the February 1950 issue of Amazing Stories. The protagonist is John Storm (no, not the Human Torch), a guy who was drummed out of the space service because he insisted Earth needed to prepare for an invasion from Galaxy X. Now, the Earth government has found evidence that Storm was right.
He's given command of a ship and sent to Jupiter to set up a base to watch for the invaders. But many in his crew still think the whole Galaxy X thing is nonsense and Storm is soon dealing with a potential mutiny. Then there's the girl who has stowed away on the ship to find a loved one who was once stranded on Jupiter. And another girl, already on Jupiter, controls an army of robots and expresses displeasure with the new arrivals.
When the aliens from Galaxy X show up, though, everyone is going to have to figure out how to work together in order to save the Earth.
I like the story. The various character developments are fairly predictable, but the tale is well-structured and the action sequences are exciting. There's a real sense of tension in the end when the surviving humans are desperately figuring out how to destroy an entire fleet of enemy ships.
Here's where the Suspension of Disbelief issue comes in for me. Jupiter in this story is a habitable planet, with apparently Earth-normal gravity and atmosphere. I have no problem with this--I've written any number of times about my enjoyment of stories in which many of the planets & moons in our Solar System are able to support life.
But the invading aliens are said to come not from another star within our galaxy, but from another galaxy entirely. Earth scientists have detected evidence of a war being fought in Galaxy X and have somehow deduced that Earth (revolving around one of millions of stars within one of millions of other galaxies) is the next logical target for whomever wins that war.
Though I still like the story, that part bothers me. It seems "wrong" even within the parameters of a universe in which Jupiter is habitable. I think maybe the key is to keep the geography of the universe consistent with reality--even if Jupiter can support life, at least its still located where it is supposed to be. That helps establish an acceptable plateau for suspending our disbelief.
But the invaders coming from another galaxy and targeting Earth for no apparent reason pretty much ignores basic layout of the universe. That's why it bothers me.
But that's just me. For other readers, the Galaxy X issue might be just as acceptable as living on the surface of a gas giant. Certainly, its no less unrealistic when compared to real life.
Besides, "Galaxy X" has a better ring to it than "Planet X," so maybe it's all just as well.