What are the major motifs and symbols in The Great Gatsby?

Certainly, the green light at the end of Daisy's dock is a major symbol in the text. For Gatsby, it seems to represent the life of possibility that he envisions with Daisy: their love, their money, and his dream of being together. He imagines that they will want for nothing, that they will finally be able to be together with no strings attached and nothing detracting from their love. It is, in many ways, symbolic of the idea of the American Dream in general; however, the fact that it is attached to the property of people who have never had to work or struggle or persevere against any odds seems to indicate that such a Dream is an illusion.
The billboard promoting the optometry practice of Dr. T.J. Eckleburg also seems to be a symbol. George Wilson seems to think of those big eyes as being God, and -- in a way -- money does seem like a god in this society. Money seems to determine who gets to be successful or popular, who gets what they want and who does not. People worship it, so to speak. There does not seem to be some higher power watching over people, answering prayers, or ensuring that people do not suffer needlessly; in fact, money seems to be the highest power there is, symbolized by the fact that an advertisement designed to increase revenue is conflated with God in the horrible and tragic valley of ashes.