This is a great question. There are many humans flaws in the book. One of the reasons why the book is so powerful is because these flaws are very tragic, and we can relate in many ways.
First, there is racism. Crooks is the lone black man on the ranch. He lives a life of separation from the others. There is no community or friendship for him. Hence, a flaw in the book is that people cannot see beyond race.
Second, there is sexism. Curley's wife is not even given a name. She is merely known as Curley's wife. And when she is introduced, the men say that she has the "eye." Her characterization is negative right from the beginning.
Third, there is a sad utilitarian view of humanity. Once a person has given and there is little else to give, they are expendable. This is how Candy, in his old age, feels.
Fourth, there is a lack of trust among people. No one really trusts anyone else. The idea of friendship is absent. The only friends in the book are Lennie and George.
There are many more examples, but these should get you thinking.