It is hard for me to address this because I think the flaws that the character possess in the book are ones all of us have. In critiquing them, we are really critiquing ourselves. Having said that, I think that some of the characters possess tragic flaws. Certainly, Lenny's flaw is his complete and absolute faith in George. Lenny's friendship with George is predicated on a foundation of dependence on George. His childlike dream of owning a farm with animals on it is his animating spirit, his driving force. In Lenny's world, he doesn't understand the duplicitous and inauthentic propensity that exists in human beings. Perhaps, this is why is so completely tragic. We want to identify with him because in identifying with Lenny's dreams, it represents hopes for us. When he dies, dreams seem to die with him.
I think that George's flaw is that he is crushed by the weight of his dreams. In his desire to develop a life for his own self, where he is able to go to a ball game and enjoy the elements that come with a life of relative security, George believes that his dreams will come true, believes that there is a way to angle things so that he can come out ahead in the end. He is devoted to Lennie. He will get much criticism for his actions towards Lennie in the end, and I think one can go two ways on it. If it is believed that he was devoted to Lennie, then his actions were to spare Lennie a death that was going to be unkind at the hands of the lynch mob. It is believed that he wanted out of his relationship of taking care of Lennie, then he killed him for his own benefit, for expediency's sake. I think that you can go either way on it, depending on what you take from the novel as evidence of his characterization. In the end, George becomes a realist and understands his dreams are only that- dreams. This might further either reading of him.
Curly's wife is interesting. I think her flaws are two. The first is that she is in a loveless marriage. It might not be her fault, but she certainly could have done more the ensure that this marriage had some foundation, and that she can do less to antagonize Curly with her flirtatious ways. She's flawed because she is unable to articulate and change her joyless life as a wife. She also lacks the courage to save Lennie from the mob, preying on his handicap and playing to Curly's anger and irrational ways.
For some of the novel's characters, I think you could argue that Steinbeck would not describe their characteristics as flaws but rather as the consequence of living a difficult life. Here are several examples.
Curley's Wife--she is impulsive. She marries a man whom she barely knows because she is so desperate to escape her mundane existence. Because she did not think about her actions, she ends up in a miserable, abusive marriage, in which life is even more tedious than it was before she ran off with Curley. She is also selfish because she knows that she is bringing trouble to any man she talks to, and yet she risks it anyway to get the attention she so desires.
Curley--he has "little man syndrome." Steinbeck's description of Curley (through Candy) is that Curley is a small man who wears high-heeled boots. Curley's insecurity about his height and possibly also his desire to do something other than ranch work cause him to bully others. Almost all of his actions stem from his attempt to make himself "bigger." He verbally and physically attacks bigger men (Lennie, Carlson, Slim) to show that the is not intimidated by their size--for him, this means that he portrays himself as their equal. He uses his position on the ranch to make others think that he is more important than he is.
George--George's flaw is impatience; although, if I had to endure what George has gone through, I would be impatient too. George is impatient with life. He wants the farm dream to come true now; he's tired of waiting for it and simply talking about it. He's impatient with Lennie's inability to control himself. He's impatient with his lot in life but sometimes fails to realize that his lot is not much different from the other ranchhands' lives.
These are just a couple of examples. For the rest of the characters, you should be able to come up with personality traits that might be characterized as flaws. I do think, however, that you might find it difficult to find anything that Steinbeck intended to be a "flaw" in Slim.