In what ways were the 1920s a reaction against the progressive idealism that held sway before and during WWI?
The 1920's were not only a time of fun and frolic; but a time of entrenched Nationalism and a belief that the country should go "back to the basics." The 1920's saw the rebirth of the Ku Klux Klan which now exerted its influence nationwide. Rather than opposition to Black equality; the Klan now dedicated itself to "100% Americanism." Also, the pseudoscience of Eugenics became popular, and immigration was severely limited. There was a "Red Scare" in which anyone who questioned the government was suspect. Then too, there was a return to Fundamentalist religion and an overt attack on the work of Charles Darwin.
Also, the horror of the war created tremendous disillusion among writers and intellectuals. A number of them, such as Ernest Hemingway and T.S. Elliot were so alienated that they left the U.S. for Europe. They were designated by Gertrude Stein as the "lost generation.
In some ways similar to what Pohnpei posted, the 1920s were filled with reactions to WWI that often had a great deal to do with the idea that there was so much evil and destruction in the world and no amount of piety or religious fervor was going to fix that.
If you look at much of the art and literature of the time, there was a distinct reaction to the terror and the horror of WWI and even in the US where we escaped all of the physical destruction of cities and our landscape, there was still a reaction to the stories the soldiers would tell on returning home and the many disabled (physically and mentally) veterans that returned from the front.
And in many cases the reaction was to turn to fun and hedonistic pursuits as a way of assuaging or fighting back against that pain and darkness.
The best way to put this is that the 1920s were a time when people stopped being idealistic and started being much more interested in hedonistic things -- in having fun and in getting ahead financially.
Before WWI, there was a Progressive movement that was, as you say, very idealistic. Progressives were trying to change the world -- perhaps trying to perfect it. A perfect example of this was their attempt to legislate morality through Prohibition.
After WWI, this idealism seemed to be gone, perhaps because of how terrible the war had been. You can argue that people no longer thought the world could be perfected so they just wanted to turn their attention to having fun.
Because of this change, the '20s were a time of fun and materialism. People were getting rich because of the stock market boom. They were enjoying themselves with the new movies, the plentiful cars, and the exciting new music. They were coming to have different values -- ones that emphasized having fun.
Overall, then, after being idealistic before WWI, Americans turned towards having fun and getting rich in the 1920s.