Of course there will be differences between a novel and a movie made from that novel; the question for readers is always what the director will choose to omit, add, or change. In the case of Ethan Frome by Edith Wharton, the movie is a semi-accurate but tepid portrayal of both the story and the meaning of the story. The movie does contain small changes or omissions which do not seem to make much difference in the narrative (in which case, why they were changed?). In the movie, for example, the narrator/storyteller is a preacher rather than an engineer. The engineer is better when we read because then we see, at least a little, what Ethan might have become; in the movie, that is not really a focus.
Some similarities between the movie and the novel include some big-picture things. For example, the stark, northeastern setting in the novel is expertly portrayed in the movie, as are the unique colloquial accents the characters speak in (which we tend to subconsciously "standardize" as we read to ourselves in our own voices). Zeena is just as revolting in the movie as she is in the novel; it is easy to see why Ethan is drawn to Mattie. (Unfortunately, in the movie this is based more on attractiveness than on the fact that Zeena is mean and Mattie assuages his loneliness and desire for true companionship). There is a sledding accident in both the movie and the novel, and of course that leads to the great reversal between Zeena and Mattie in the end. Unfortunately, this specific event is also on my list of differences between book and movie (details below).
The most disheartening (to me, as a teacher and lover of this novel) difference between these two forms of the story is the fact that in the movie Ethan and Mattie physically consummate their relationship. That is absolutely antithetical to the novel. Even on the night Zeena is gone, they never touch. After Mattie goes to bed for the night, Ethan says:
When the door of her room had closed on her he remembered that he had not even touched her hand.
What this change does is make the relationship more physical than emotional, as I mentioned above. Ethan does not choose anyone because they are better than Zeena; he chooses Mattie because she brings color and warmth into his life, not because she is young and pretty.
The coming to his house of hopeful young life was like the lighting of a fire on a cold hearth.
In the novel Mattie is inept at many things, and Ethan (perhaps the most inept of all) feels like he is a master around her without feeling the need to master her.
Second, the sledding incident in the movie is diluted. In the movie, all Mattie says is that she “doesn’t want to ever leave this mountain”; however, it is clear in the novel that Mattie is the one who wants their lives to end while they are together.
"Ethan! Ethan! I want you to take me down again!"
"The coast. Right off," she panted. "So 't we'll never come up any more."
"Matt! What on earth do you mean?"
She put her lips close against his ear to say: "Right into the big elm. You said you could. So 't we'd never have to leave each other any more."
The movie does not fairly represent the novel in this aspect.
Finally, Ethan setting out poison for the foxes who are after his chickens is a gratuitous foreshadowing of the later suicide, and it never happened in the novel.
In short, what the movie does not capture is the romantic longing which permeates the novel; those who know the novel miss that.