There's "disappointed", and then there's "betrayed".
I worry about my audience's expectations all the time, whether I'm in Scrivener or in the bedroom. I worry about it in regards to story because I know what it feels like to be disappointed with where a story goes.
I also know that sometimes it can go deeper than disappointed. Sometimes, if the hook is set real deep at the beginning, and you really want to go where you think the author is taking you, and then they pull a fast one, you're left feeling like a used idiot whore in a prom dress.
In creative work, we are gardeners, pruning and curating a relationship with our audience that we only half control but into which we pump 90% of the energy. Without that audience (even if it's just ourselves), our work has no purpose, and it can never be complete, in the tree-falling-in-the-forest sense. The final ingredient is always the audience, and while it's not helpful to pander to the audience, it is helpful to cultivate that relationship, and to be aware when choices you make might endanger it.
It's okay to do things that I, as an audience member, don't expect. It's okay, and even desirable, to play games with my assumptions and to assert your power over the direction of the storytelling. But you can go too far, and to introduce change and misdirection that breaks the spine of the relationship between me and the work. Usually, this is because something's been done that has made me think of You, the Artist, wielding "art" with too heavy a hand.
When this happens, I feel that a trust has been betrayed, that the promise of goods and services to be rendered has been reneged upon, and that my purpose for beginning this journey no longer applies, and that I have been had.
This is often described as "losing the reader". But I prefer to think of it as the artist breaking the contract with the audience. The previous parts of the piece should function as an assemblage of expectations that, while none are completely keystone, together amount to what I as an audience member agree that I want to experience or learn or feel.
A recent example of when an work broke a contract with me: Get Low.
The film is not a bad film. It's structure is valid, the performances are good, and the overall story is somewhat compelling. But the nature of the mystery and the atmosphere and the clues that are established in the film's first two acts are (I believe) sharply at odds with what is finally revealed during Robert Duvall's (poorly photographed) final monologue, wherein all of the answers are artlessly (from a storytelling point of view) flopped out onto the table, in complete disregard for our expectations of how backwoods mystery stories are supposed to be told.
In retrospect, the film could actually be said to simply cultivate confusion until Duvall is ready for his close-up. And then it tells its answers instead of showing them, and we're all supposed to go home, like that's what anyone signed up for.
The contract with the audience is signed while the story is unspooling from the point-of-view of the funeral assistant, who lives in a town with other citizens, and the story seems to be about the intersection between human myth and reality. But after the hook is set, everyone is forgotten about except for the hermit, with whom we are unprepared to relate BECAUSE HE'S A FUCKING HERMIT.
(And am I the only one who doubts that a wad of forty-year-old bank notes would still be legal tender? Thus puncturing the motivation of the first act?)
So I was betrayed. And I beg you all: it is better to disappoint your audience, to wear your shittiness on your sleeve, than to disguise it behind convoluted artifice in the hope that you can emulate great art from the outside in.
I am not condemning the use of ambiguity and opaqueness for the purposes of storytelling. I am condemning purposeful misdirection in the pursuit if hiding your story's Aboutness, upon which your contract with your audience is being signed at every moment. Be conscious of this, and do not underestimate the power of the contract to make me happy, or to make me mad.