What are three major themes present in The Great Gatsby?
"Moral corruption and privilege", taken together, can be seen as one major theme in the novel. The wealthy characters, despite claims of being "sophisticated", are almost universally moral failures. Daisy, Tom, Gatsby and Jordan are each cheaters in one way or another. The wealth and power these characters possess functions as a means to justify or excuse their bad behavior, or, seen differently, their wealth removes them from moral responsibility.
There are no spiritual values in a place where money reigns...
"Dreams, dreaming, idealism and illusion" can be taken as another major theme. Gatsby pursues an ideal and this pursuit effectively animates the novel. The ideal that Gatsby pursues is associated with profound naivety and self-confidence, as we see in one of the novel's more well-known moments.
Concerning his behavior with Daisy, Nick tells him he can't repeat the past. “Can't repeat the past,” Gatsby replies, “Why of course you can!”
Another major theme relates to "class conflict and materialism". In various ways, the novel explores exploitation of the poor and super-valuation of material possessions (cars, houses, and even dogs), as well as a quite superficial value system (as displayed by Gatsby's party guests).