What type of flaws do each of the characters from Of Mice and Men have?

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.