In the European medieval period, all the kids were going nuts for a type of drama called a morality play, in which the hero (often a guy named, cleverly, Everyman) encounters various human attributes in the guise of people trying to convince him to stay on the path to God. So, for example, a character named Vice might try to tempt Everyman away from God, while a character named Justice might remind him that he really wants to stay on God's good side.
Of Mice and Men like a straight allegory, but it still has the quality of a morality play. George is our Everyman, and the ranch is populated by people who are as much representations of groups of people or attitudes toward life as they are character in themselves. Let's take a look at our handy-dandy list:
Read as a morality play, Of Mice and Men stops being a Depression-era slice of really depression (LOL) life, and more of a universal story about how to be a man. (And how not to be a woman.)