How does Nick change throughout the novel The Great Gatsby?

At the beginning of Nick's reminiscence of the summer he met Gastby, he has "small-town syndrome."   He had just returned to Middle America (America's heartland and the center of conservative living) from WWI, where he had glimpsed everything from freedom to death.  His horizons had been broadened significantly, so when he returned after the war, he felt stifled in the Midwest; thus his longing for the decadent and fantastic lifestyle of New York, but the problem with the fantastic is that it rarely has anything to offer beneath the surface. 

When he first arrives in New York, Nick is fascinated by the lives of the wealthy and the freedom they embody (including freedom from responsibility, evidently).  However, as the novel progresses, he sees the impact of this behavior on the lives of others; he recognizes the atrocities that the elite of society commit toward those they consider beneath them (i.e. Tom's abuse of Myrtle Wilson; Tom's treatment of George Wilson; Tom and Daisy's method of dealing with Daisy killing Myrtle; Tom ultimately setting Gatsby up to be killed and not feeling any remorse).

By his thirtieth birthday, Nick realizes that this crazy, superficial lifestyle is not what he desires at all, and that he misses the wholesomeness of the Middlewest.  In this sense, Nick becomes rather representative of the 1920s: the turmoil and free living of the early part of the decade leading into the conservative 1930s.