What are the major conflicts (man v. himself, man v. man, man v. nature, man v. society) in Of Mice and Men?

This is a good question. The whole book is filled with conflict. This is why the book is so tragic. 

First, we have the conflict between the rich, who own land, and the day workers. The poverty of workers like Lennie and George is great. They really have nothing, whereas those with wealth are far better off, and they have power. Moreover, they use this power to exploit the laborers. 

Second, even among the workers there is conflict. For example, when George and Lennie arrive at the ranch, they have to make a place for themselves. No one really accepts Lennie; they tolerate him as long as he keeps to himself. Curley is downright hostile towards Lennie. Candy is always filled with insecurity because of his age, and Crooks, as a black man, always feels alienated. 

Third, there is also conflict among the sexes. Curley's wife is not even given a name. She also feels alienated from the men, as they avoid her. She is almost made to feel non-existent.  

Fourth, there is also conflict within. This comes out when George has to make a decision of what to do with Lennie when he accidentally kills Curley's wife. In the end, he shoots him.