Slim reflects the hard-working ideal that is American exceptionalism. He does not raise his voice, lose his temper, or act mean toward anyone. Instead, he treats other characters with due respect, shows appropriate interest in people, and acts with moral responsibility. He is also considered a thinking man, something that may have been a little uncommon for Depression-era ranch hands.
His character is carefully depicted through a visual description in chapter 2:
When he had finished combing his hair he moved into the room, and he moved with a majesty only achieved by royalty and master craftsman. 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. (Chap. 2)
This is a man who is presented as respected by the other characters for his strength, ability, leadership and hard work. Furthermore, he was valued for thought:
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. (Chap. 2)
Steinbeck's presentation of Slim seeks to ensure readers understand he is a character worth paying attention to and worth admiring.