Based on what she tells us in the novel's introduction, what is Edith Wharton presuming about her readers, and providing for them in Ethan Frome?
In the Introduction to Ethan Frome, Edith Wharton says: "Every novelist, again, who 'intends upon' his art, has lit upon such subjects, and been fascinated by the difficulty of presenting them in the fullest relief, yet without an added ornament, or a trick of drapery of lightning. This was my task, if I were to tell the story of Ethan Frome; and my scheme of construction - which met with the immediate and unqualified disapproval of the few friends to whom I tentatively outlined it - I still think justified in the given case. It appears to me, indeed, that, while an air of artificiality is lent to a tale of complex and sophisticated people which the novelist causes to be guessed at and interpreted by any mere looker-on, there need be no such drawback if the looker-on is sophisticated, and the people he interprets are simple. If he is capable of seeing all around them, no violence is done to probability in allowing him to exercise this faculty; it is natural enough that he should act as the sympathizing intermediary between his rudimentary characters and the more complicated minds to whom he is trying to present them. But this is all self-evident, and needs explaining only to those who have never thought of fiction as an art of composition."
In her introduction, Wharton explains that sometimes a story doesn't have to have complicated characters and a deeply symbolic storyline in order to make a point. This idea was met with some criticism from her peers and other fellow authors, but she stuck to it, and created Ethan Frome from it. She assumes, in writing this novel, that her readers are complicated, deep, thoughtful individuals who are capable of analyzing literature and forming conclusions about it. She also assumes that because we are thinkers, she can create characters who are simple and uncomplicated; they don't need to be super complex and difficult to understand. Because we ourselves are complex, we impose our own complicated interpretations on things; the characters merely need to provide a platform for that interpretation.
So, in Ethan Frome, Wharton paints pretty straightforward characthers--the duty-bound and lonely Ethan Frome, the manipulative and bitter Zeena, and the naive and innocently beautiful Mattie. It is upon these straightforward and simple characters that Wharton trusts we, as the thinking reader, will be able to draw conclusions about. She provides a simple storyline that is the foundation for a more complex moral and character discussion. I hope that those thoughts helped; good luck!