Unless absolutely necessary you will have a positive feeling to your story. By this we don’t mean it will have a happy ending or that we expect pollyanish sentiments out of you. Your novel and setting can be as dystopic as you want it. In fact, your character can die at the end. Just make sure he goes down fighting and dies for something, so the reader doesn’t feel cheated.
If I had to condense all of my expectations about a book, any book, into a single sentiment, it would be this: I want to come out of that book knowing something, however minor, that I didn't know going in.
I'm not talking about technical knowledge, although that can be interesting. I'm currently reading Fuminori Nakamura's The Thief, which seems to be exploring some of the same territory as Robert Bresson's Pickpocket. Both stories give you a great deal of detail about the moment-to-moment activities of a man engaged in petty crime, but they don't make the mistake of confusing the technical detail of his work with a story about him. There is something both stories have above and beyond that, about human nature — and in my purview, human nature is really the only subject worth writing about for a human audience.
The best way to not cheat an audience, then, is to write something insightful about human behavior. The problem is that many authors either ignore this entirely (because they're writing SF, dammit) or supply us with human behavior that is so hopelessly entrenched in the mundane that there barely seems a reason to write a book in the first place.
(Warning: Digressions ahead.)
I'll start with the first problem. Some people, I suspect, would find fault with the idea of an SF (or even fantasy) story needing to revolve around such a thing. They're literatures of ideas or imagination, they claim, and all this piddling about with human nature is so ... quotidian. This manifests most visibly in the screeds I've seen posted by some SF/fantasy authors (I might be guilty of this myself) where they abhor the boringness of literature that is about mingy little people leading mingy little lives. The here and now is a prison to be escaped from. They want excitement, escape, adventure ... but I thought a Jedi craves not these things?
The escapist impulse, as I call it, is not by itself the problem. There is nothing wrong with wanting to look past our moment in time and our place in the world, and yearn for (and maybe imagine) something different. I don't doubt for a second that the present moment always looks (and feels, smells, tastes) like a prison to those trapped in it. But all of that imagination, all of that yearning, is rooted in who we are and what we do right here and now. It cannot be any other way, and the best thing to do with such an understanding is pay respect to it instead of turning your back on it.
One of the unforeseen consequences of total escapism is that precise and insightful observation of human nature is one of the many babies that gets pitched out with the bathwater of realism. Realism does not have to be a synonym for "boring". A great deal of realistic literature is aggressively flat (see the gray goo problem), but people freely ascribe that limitation to realism itself instead of overzealousness on the part of the author. The end result is the thinness of so much SF&F — good for a lark, and if that's the only kind of reading you do, you won't notice anything's missing. But there could be more, and not simply in the sense of an experiment that goes over everyone's heads.
On a related note, I suspect a great many writers of serious fiction simply find the mere idea of SF or fantasy inherently silly, and I wonder if that's because they have been bombarded with so many lousy examples of it. I know I'd feel that way if my sole exposure to SF&F consisted of The Wheel of Time and Baen/Tor-style gung-ho military SF. But I also wonder if it has roots in other things. Pauline Kael had this problem as a critic, and owned up to it: she had trouble seeing any SF&F movie without wanting to giggle.
The more I delved into that impulse, the more I was reminded of Nietzsche's aphorism that anything that makes you laugh is a sign of an emotion that's died. If someone finds SF or fantasy silly, or vaguely stupid, that strikes me as a symptom of the death of a part of that person's imagination. I know someone who has tried to read SF&F but always gives up, and when I pressed them for more on the subject they simply replied "I have no imagination." I was tempted to reply, "Well, it's the author that's doing most of the imagining here, you're just along for the ride," but a) I knew that wasn't a complete picture of the relationship between the author and the reader (in my mind it's closer to an on-the-spot collaboration, where your imagination is enlisted in the service of his), and b) that was beside the point. If someone has no interest in having their imagination enlisted in certain ways, there's very little you can do to tempt them into following along.
It's all the more dismaying when it's a writer that expresses such things. You would think that if there was any one kind of person on the face of the earth who didn't straitjacket their imaginations, it would be a fellow writer. Sadly, it happens with writers all the time. I have met far too many writers of one kind or another who spit on the people one genre to the left of them, who sneer at the idea of "serious" writing taking place anywhere but on their desk, who in short go out of their way to be venomously parochial. I cannot see anything in such behavior other than the senseless fear of someone protecting their turf.
That brings me to the other half of the problem — the authors of the Really Real Realism, who more often than not seem to be compulsively disinterested in escape literature of any kind. Part of me believes in a very simple reason, almost idiotically so, for this: they simply haven't read it. They do not know what it can do at its best, because they are surrounded by too many examples of it at its worst, and do not see the point in investing the effort to find out otherwise. Well, sure. It's a big world with a lot of books in it, and there's only so much you can read (or want to read) in a lifetime. But again, the mingy-mindedness of such an attitude is dismaying — especially when it comes from the very class of people you think would be most interested in opening their eyes and broadening their minds.
In both cases, we have two different ways the reader can feel cheated. In the first, with SF&F, character (and sometimes story) goes missing in favor of conceits and concepts. In the second, with mainstream lit, the imagination SF&F taps into gets ground underfoot and discarded, or — even worse — caricatured. In other words, neither one has the monopoly on leaving the reader feel unfulfilled.
The choice should not be between a) the grungy flatness of people moping in kitchens and drinking beer out of long-necked bottles or b) far-flung epic adventure that mistakenly substitutes the easy shorthand of its genre for real life observed and understood. Both are just different ways to fall short. Each has something to teach the other, if only they will listen.