As I mentioned in Write What Readers Love, I read tons of reviews on books that fall into the same genre I write in. Even if I've never read the book itself, I receive huge insight into reader expectations by reading reviews and I've noticed some trends.
This top five list isn't a result of polling hundreds of readers - at least not officially. But it is based off three star and below reviews from across a spectrum of indie-pubbed SciFi books. Perhaps reader objections are different for traditionally published SciFi, but I doubt it. If anything, I would think readers tend to give indie-authors a bit of a pass because "it comes with the territory of reading indie books."
That might sound like I'm advocating sub-par writing, formatting, or editing in indie-pub but that is far from the case. I'm not condoning anything less than the very best an author can do, but I do think those who are less discriminatory easily entertained don't mind as much the need for a better content editor, or the occasional mis-used word.
That being said, editing issues aside, here are the top five mistakes SciFi Indie authors can make (in no particular order.) Think about your work. Do any of these apply?
1. Lack of Plausibility: How far does the reader have to suspend belief? Readers of science fiction expect to be dealing with something that is a product of your imagination. Suspension of belief is expected. But writing science fiction doesn't give you free reign to go against human nature or to defy the rules you've created for your world.
Typically, plausability issues in science fiction deal more plot or world-building than it does with characters. I suspect this is the case because humanity (or at least something like humanity) remains constant, despite other planets, worlds, or twilight zone scenarios.
We have to believe that the rules set within your world make sense with each other, that they don't contradict. We might not know what all those rules are. We may not understand how your world came to be or how your machines work. In fact, a lot of times, half the enjoyment of reading scifi is wondering how something came to be (and hoping that the author answers those questions later). But we have to have some rules as a frame of reference, and those rules must make sense among themselves. Think of it as the principle of non-contradiction for writers.
If your beta readers are commenting that such and such is extremely unlikely or would never happen or doesn't make sense in the world you've presented, that is a huge red flag. But you are fortunate that they caught it before your book went on the market and garnered a bunch of three star reviews for the same reasons.
2. Filler: A cardinal rule of good writing is that every sentence in the book has to expose character, move the plot forward, or set the scene. But the best scenario is when it does all three at once. And the worst?When it doesn't do any of those.
Readers hate it when an author includes eight pages worth of writing that could easily have been left out, e.g. battles that don't have any affect on the plot, extensive writing on - at best - side characters that have nothing to do with the main story, main characters musing about things that have no effect on their decisions or don't have anything to do with anything.
Don't be the kind of writer that includes multiple scenes that you wrote because they were cool. Your book is not a collection of cool scenes (unless it's sold as a collection of cool scenes). It's a book, with a beginning, middle, and end. Deviate from that and you'll have some peeved customers.
3. Lack of Character growth: Characters are the heart of a good story. One could have a great plot and great setting but without characters that have complex motivations and personalities, your readers will lose interest - and fast. They might not evolve and develop, but they should at least have multiple motivating factors and a well-rounded backstory.
If your characters aren't real to you, if you can't see them in your mind's eye, like the way you can "see" a friend or enemy, they won't be real to your readers either.
Think about how varied the life is in your social circle, how each person you know is affected by their temperament, their upbringing, and their life experiences. Now look at your characters. Do you understand them as well? If you don't, then they need work before you continue writing your story. You might discover that your plot doesn't work with the character you are envisioning. Or worse, that you've written a character that is completely superfluous to your plot. Your main character should be intrinsic to the story and drive the plot.
If you need help, try this list from Plot to Punctuation. There are tons of other character development tips available online, so I suggest looking around and finding something that inspires you to get to know your character better.
4. Insufficient World Building/Backstory: There are a lot of different reasons why scifi readers love scifi. One of the biggest attractions is the originality of the world presented. Many of the three star and below reviewers expressed disappointment in author worlds not fully explored.
Why is world building so difficult? Because we are so accustomed to the knowledge we have, that when we try to break it down into its minute parts, we miss a lot. A good back story, an understanding of all aspects of your world and why, and the why behind the why, doesn't make it into your book. At least not the majority of it. What it allows for is a plausible world, something that seems organic and real.
That doesn't mean give us three pages of narration about what this world is like, but it does mean we will be more convinced of the plausibility of your world. Your greater knowledge will affect how you tell the story and what your characters do and how they do it
If you take the time (and perhaps utilize some world-building checklists) before you start on your plot, you'll have a much better chance of your story being fully developed.
5. Too troped/too cliched: Perhaps more so than other genres, scifi readers want originality - if not in story line, than in characters, if not in characters than in setting. Something should set your novel apart from other novels. If the three major aspects of your book, setting, plot, and character all follow overdone tropes, reader attraction will lag.
Of course, tropes are tropes with good reason. We are attracted to certain things: the underdog, the adventure story, the explorer turned battling hero. But if there is zero originality, one can't expect to be getting reviews above four stars very much. If you combine overdone tropes with any of the four flaws above, you're looking at a book that will die a literary death in a short amount of time.
TVTrope's Grand List of Overused Science Fiction Cliches is extensive (and amusing). If you are wanting to find a way to work yourself out a cliche, try throwing in the opposite of what the reader would expect. That might seem like an easy fix, but it has successfully worked (e.g. Columbo). You can also check out this list here for some ideas on how to make your story unique.
All of the above is based off of extensive reading of "how to" books online and scifi reader reviews. If you think any of this information is suspect, I invite you to do the reading yourself. In fact, I heavily encourage that you read reviews on books similar to yours in order to be able to avoid specific pitfalls in plot device, character development, world building, etc. that your niche is susceptible to. And keep in mind that reviewers aren't going to be any nicer to your book than they are to anyone else's, but if you give readers what they want, you have a much higher chance of becoming a best seller.