The reader needs to be aware that Nick is the narrator, as well as one of the most important characters. Since the story is told through his eyes about people close to him, we cannot be sure that the impressions he gets are necessarily accurate. And anything he says about himself cannot be taken as gospel in particular. So whatever conclusions the text gives of a character, remember the person that they are being filtered through.
Nick importantly brings up that he is from the midwest. Toward the end of the book he says that all of the characters were not from the east and therefore not fit to live there. Their past lives hindered their ability to live in their current ones. He also says that he is from the middle class. Class is one of them most important themes in the novel and affects the relationships of the characters. Much of the way people are treated can be linked to their class and social position.
Take Daisy, for example. While Nick and Jordan are their for dinner she talks about having her little girl. She was very upset after the birth because Tom was nowhere to be found. When the nurse tells her she is a girl Daisy said " I'm glad it's a girl, And I hope she'll be a fool- that's the best thing a girl can be in this world, a beautiful little fool". The social position of women is criticized here. Daisy is a witty and clever woman, far more so than her brute husband. Yet she is the one treated as inferior because she is a woman. This is also why Tom can flaunt his affair. He does not have to worry about the consequences.
Later, when Nick sees Gatsby on the lawn, he watches him intently. He notices that Gatsby is looking out at the water but the only thing that is visible is a green light. That green light is the most important piece of symbolism in the book. It is a light on the end of Daisy's dock. It's meaning and references in the text are discussed in the symbolism section of this site.