There are several reasons why Slim is respected on the ranch. Perhaps the most important one is that he is a skilled worker while all the other men are just common unskilled laborers. Slim is introduced in Chapter Two.
When he had finished combing his hair he moved into the room, and he moved with a majesty only achieved by royalty and master craftsmen. He was a jerkline skinner, the prince of the ranch, capable of driving ten, sixteen, even twenty mules with a single line to the leaders. He was capable of killing a fly on the wheeler's butt with a bull ship without touching the mule. There was a gravity in his manner and a quiet so profound that all talk stopped when he spoke. His authority was so great that his word was taken on any subject, be it politics or love. This was Slim, the jerkline skinner.
So Slim is respected because of his skill but also because of his air of majesty which comes from being skilled and indispensable to the employer. He lives in the bunkhouse with the other men and uses the same kind of poor grammar, but he is undoubtedly the authority figure, both out in the fields and in the bunkhouse. Since Slim is in charge of the whole operation when the men are working in the fields, it is natural that he should continue to be in charge in the bunkhouse. He and George seem to strike it off pretty well from the beginning. This is mainly because both men are intelligent and realize they are superior to the other men in that respect. Slim is important to the story in many ways. One way is that George trusts him and confides in him. Slim takes charge when Lennie crushes Curley's hand, and Slim persuades Curley to pretend it was a machinery accident in order to protect George and Lennie from getting fired. Curley could easily get them fired or could even fire them himself. Slim comforts George after George shoots Lennie and takes George away to buy him a drink. Slim dominates by his personality, intelligence, understanding, and kindness, in sharp contrast to Curley who thinks he is the boss and gets no respect because everybody senses his deep feelings of inferiority. The reader gets the impression that Slim is a father figure. He looks after his men. They appreciate this, because they are not used to having anyone look after them or care anything about them.