Ursula K. Le Guin is an award-winning science-fiction and fantasy author, so the tone of the story "The Ones Who Walk Away From Omelas" is poetic, almost as a fable. The opening lines set the stage:
With a clamor of bells that set the swallows soaring, the Festival of Summer came to the city Omelas, bright-towered by the sea. The ringing of the boats in harbor sparkled with flags. In the streets between houses with red roofs and painted walls, between old moss-grown gardens and under avenues of trees, past great parks and public buildings, processions moved.
Using powerful description, Le Guin shows an idyllic city of happy, content people, people who enjoy prosperity and life, with few dismal thoughts. Later in the story, the narrator explains that the world of Omelas is not set in stone:
Perhaps it would be best if you imagined it as your own fancy bids, assuming it will rise to the occasion, for certainly I cannot suit you all. For instance, how about technology? ...they could perfectly well have central heating, subway trains, washing machines, and all kinds of marvelous devices not yet invented here, floating light-sources, fuelless power, a cure for the common cold. Or they could have none of that: it doesn’t matter. As you like it.
This allows the story to have more of a fairy-tale aspect, instead of a hard-and-fast solid world. The narrator is more concerned with the sense of the world, the feel of the people who live their happy lives with the knowledge that their happiness depends on someone else's suffering. The reader can then focus a personal sense of "ultimate happiness" on Omelas before the twist in the story comes:
In the room, a child is sitting. It could be a boy or a girl. It looks about six, but actually is nearly ten. It is feeble-minded. Perhaps it was born defective, or perhaps it has become imbecile through fear, malnutrition, and neglect.
The child used to scream for help at night, and cry a good deal, but now it only makes a kind of whining, "eh-haa, eh-haa," and it speaks less and less often. It is so thin there are no calves to its legs; its belly protrudes; it lives on a half-bowl of corn meal and grease a day. It is naked. Its buttocks and thighs are a mass of festered sores, as it sits in its own excrement continually.
They all know it is there, all the people of Omelas. Some of them have come to see it, others are content merely to know it is there. They all know that it has to be there.
(Le Guin, "The Ones Who Walk...," liferoar.wordpress.com)
The tone changes abruptly to flat, simple descriptions, showing that however the outside is glorious, the inside of this room never changes. It is this place and its horrors that allow the outside utopia to exist. This above all else is the only concrete thing about Omelas; whatever else is "imagined" above, it is dependent on this single moral choice.
The last line -- "But they seem to know where they are going, the ones who walk away from Omelas." -- returns to the poetic form, and admits that there might be nowhere to go for the people who have lived there. Just like the reader imagines Omelas, the escape must also be imagined, because once the knowledge of the starving child is known, it cannot be forgotten.