The Midwest is the source of truth, honesty, purity, and uncorrupted living. Nick Carraway was eager to escape his family roots in the Midwest at the beginning of the story because "the Middle West now seemed like the ragged edge of the universe", so the opportunity to relocate to the excitement and sophistication of the East was very attractive to him.
Through the course of the story, however, Nick comes to understand how superficial lives and relationships are among the characters he meets in the East. He comes to consider the values of Easterners to be uncaring of anything except self-protection and personal gratification. Nick discovers that he is not capable or willing to adopt that type of attitude and conduct in his life.
That's my Middle West...a little solemn with the feel of those long winters, a little complacent from growing up in the Carraway house in a city where dwellings are still called through decades by a family's name.
All of the primary characters were originally from the Midwest. When they meet again in the east (New York City/Long Island- just about the farthest east you can get in the U.S.), they are living lives that pervert and subvert Midwestern values.
For example, Nick's family is in the hardware business. They make their money from literal heaviness. The source of their wealth is concrete and substantial. When Nick leaves for his life in New York, he enters the bond business. The source of his income (he never generates any real wealth) is literally as light as paper. Bonds are paper instruments that represent interests in a company, but by becoming a bondsman, Nick is trading these pieces of paper based on promises of money. He doesn't actually produce anything.
Towards the end of the novel, Nick reminisces about train rides home to the Midwest. He waxes poetic thinking about his homeland as something welcoming and substantial. The east, after all, is only potential- the great green expanse for the first Dutch settlers.
The Midwest represents normalcy and traditionalism to Nick as opposed to the decadence, immorality and a "quality of distortion" he finds in the East. He writes near the end of the novel at some length of "vivid" winter memories of the Midwest. The images he uses are traditional and evocative of Currier and Ives: he talks of "the real snow, our snow" of the Midwest, of how it would "twinkle" against the train windows, of the way "a sharp, wild brace came suddenly into the air." For him, the Midwest symbolizes that which is safe and domesticated. It is street lamps and sleigh bells and the shadows of holly wreaths thrown onto the snow by lights in the windows. "I am part of that," he says, describing himself as solemn and complacent and noting that he grew up in the "Carraway" house, in a place traditional enough for homes to be known by their owners' names.
He notes, as well, that all of the main characters come from the "West": Daisy, Jordan, himself, Tom and Gatsby, and wonders if it isn't "some deficiency" they share as Westerners that makes them unable to adapt to the East.
In contrast to the Midwest, Nick perceives the East as fantastic, likening it to an El Greco painting, represented by houses "at once conventional and grotesque."
But despite Nick's attempt to draw a sharp distinction between East and West, one must remember that none of these characters emerged wholly pure from their Midwest background. None were solely corrupted in the East, but brought their baggage with them. These passages point to Nick's role as artist, storyteller and myth maker.