Nearly every character is discriminated against in Of Mice and Men.
On a broad scale, all of the workers on the ranch are discriminated against. They are poor workers with little prospect of upward mobility. They are there because they have to be. They have no choices. The wealthy, who are not even mentioned, certainly look down upon them.
When it comes to people like George and Lennie, there is an extra level of discrimination, because they are migrant workers. They move from one place to another in search for work. People see them as expendable.
Curley's wife is discriminated against as well. She is ostracized by the men, and she is in a loveless marriage. More to the point, she does not even have a name. She is merely Curley's wife.
Candy is also discriminated against because of his age. Symbolically, his dog is put down owing to his old age, and this act shows that people do not really care for the elderly. What make this sadder is that the implication is people are only good for production. When productivity goes down, they are no longer important.
There is significant discrimination in the novella, against the aged, against women, and against African-Americans.
Candy is discriminated against because of his age and disability. He knows that he will be "canned soon" due to his lack of usefulness on the ranch.
Curley's wife, who just wants attention, is discriminated against due to her flirtation with the men. She is regarded as a "tramp" and a "tart," though she doesn't seem to be seeking sex: all she seems to want is companionship. Curley's wife is viewed so poorly that she doesn't even receive a name.
Crooks is threatened with lynching from Curley's wife. His only mistake was being black and daring to speak up to a white woman. He felt comfortable with Lennie and Candy and "forgot" his place. Curley's wife says that she can have him "strung up" with no trouble at all.
Many characters in Of Mice and Men are discriminated against in their society. The main characters, Lennie and George, face discrimination because they are in the working class and are poor and without land. For these reasons, they are often subject to unfeeling bosses and unfairness. They must live a migratory life, despite their dream to one day own their own farm. In addition, Lennie faces misunderstanding and discrimination because he is developmentally disabled and to some degree has the understanding of a young child. People can take advantage of him, and they often do. In addition, he does not understand his own strength, and the combination of his strength and mental disability make him frightening to others.
Candy, another worker on the ranch, faces discrimination because he is old and has lost a hand in an accident. His value comes from the work he can do on the farm, and, as he can't do as much work as the younger, fully abled men, he is less valued. Carlson, another hand on the ranch, shoots Candy's old dog, as he argues that he will put it out of its misery. Candy's dog is a symbol of the way in which the old, disabled, or infirm are not valued on the ranch.
Crooks, as stated in the question, is also left out because he is black. He tells Lennie he, Crooks, is not wanted in the bunkhouse because he is black, and he is isolated from the other men.
Finally, Curly's wife is isolated because she is a woman. She is the only woman on the ranch and has to spend her time in the house alone, where she becomes lonely. Ironically, Lennie, who has the mental state of a child, is perhaps the only character who is not judgmental of other characters. He is open minded and fair and has no preconceptions about how he should judge others.