Nick serves as a representative of every man who could peek into the world of the 1920s. Nick's not really rich, or overly popular or completely successful as a businessman. He is middle class and from a working class family in the Midwest. This is a majority of America.
The lens through which he looks helps us see characters for who they are and with appropriate and necessary judgment. Although as a narrator Nick claims he doesn't really judge people, his words are dripping with adjectives and adverbs that place value on other characters actions.
His relationship with Jordan shows a natural effort at a relationship and the bumps of a not necessarily successful dating relationship. He is a good all-American boy.