What did Nick mean when he said that there was something about Midwesterners that was defiant and thereby subtly undaptable to eastern life?
Nick definitely sees traditional Midwestern values and perceptions as being more grounded in reality than those of the East. He begins his narration of Gatsby's story with the confession that he is "inclined to reserve all judgments" regarding persons he meets, a habit he traces to his solidly Midwestern roots. He considers this to be a positive trait and realizes that not all people are so blessed.
As a result of this Midwestern outlook, Nick is unable to achieve a completely comfortable relationship with the lifestyle he finds in the East. When he settles, his house is on West Egg. Across the bay were located "the white palaces of fashionalbe East Egg" with their lifestyles based on having lots of money and plenty of time for whatever enjoyable activities they could find and very little concern about building deep relationships based on reality.
Nick recognizes the emptiness in the lives of Daisy, Jordan, and Tom. He doesn't judge them, but he is unable to identify with their attitude toward life and is unwilling to surrender himself to a similar lifestyle.
Even when the East excited me most, even when I was most keenly aware of its superiority to the bored, sprawling, swollen towns beyond the Ohio...even then it had always for me a quality of distortion.
In the end, Nick returns to the Midwest and the values and actions that are at the base of his character and his understanding of how life should be approached and lived.