America is a classless society. True or false? You'll have good support no matter which way you answer, but The Great Gatsby has a pretty clear answer: no. There's no such thing as the American Dream or the up-from-the-bootstraps self-made man. You are who you're born, and attempting to change classes just leads to tragedy. It's a pretty grim picture of American society—and life, to those who lived through World War I, could feel pretty grim indeed.
In The Great Gatsby, the only element not restricted to one class is unhappiness. All members of all classes are equally unhappy.
In The Great Gatsby, social norms are insurmountable barriers between people. Inter-class relationships are impossible.