I believe George represents the good things about America. He is a hard working, self educated man, who protects the innocent. Through hard work, he hopes to get a farm where he and his friends can live in peace and quiet and with dignity and respect.
Lennie and Candy represent the disabled and the old. They too desire to live together on the farm with George, because they also seek a version of the American Dream. Crook represents minorities in the book, and though cynical at first, he too desires to seek the peace on the farm.
And of course there is Curley and his wife, and the other hired hands, who represent everything that gets in the way of obtaining the American Dream: Lust, greed, anger, and prejudice. In the end, these forces prove to much for George to overcome and he and Lennie are forced to flee the farm. When George realizes that their dream is shattered, he uses the idea of it to make Lennie happy when he shoots him, allowing him to die happy and not by the hands of Curley.
I believe that these ideas still resonate today. The idea of the American Dream is still out there, but there is so much working against it and pulling it apart from all directions. Will it ever be obtainable again? That is the great question we as a nation must answer.
This is the Metlife commercial from the Super Bowl, I thought it captured the true American spirit quite well.