What are the positive aspects of George and Lennie's friendship in Of Mice and Men?

Although a study of Of Mice and Men has to focus on the negatives and what eventually happens to Lennie, it is unfair to say there are no positive aspects of George and Lennie's friendship. The friendship provides Lennie an opportunity to participate in a world that would otherwise be very unaccommodating to him; for George, the friendship provides a focus that takes him beyond merely day-to-day survival.
Lennie's disability is not clearly defined in the novella, but it is historically accurate to say a person with an intellectual disability at the time at which the story is set would have had very few options available to him. Before George takes Lennie under his wing, Lennie is cared for by his aunt. For the time period, that is believable. People with disabilities were frequently hidden away by family or institutionalized. (For historical context, consider President John F. Kennedy had a sister who was kept out of the public eye in an institution for most of her life—and she was from a wealthy, respected family.) When it came to dealing with the handicapped, the goal of the time period was not to provide rich stimulation; instead, the goal was "out of sight, out of mind." Once Aunt Clara dies, Lennie has no one obligated to care for him, so George's friendship truly does provide Lennie with entrance to a world that would never have accepted Lennie on his own. Even something as mundane as keeping track of his work card is too much for Lennie to handle by himself, but George makes sure Lennie has opportunities to work for pay in a time when jobs were hard to find even for those without handicaps. Without George, Lennie would have not had entry into a workforce where his tremendous strength could be viewed as an asset. With George, Lennie can fit in and even be quietly admired for his abilities instead of maligned for his disability.
For George, Lennie provides a purpose and meaning to everyday work. George and Lennie's shared dream of having a small parcel of land to call their own makes George different from the other men who drift and squander their paychecks on visits to bars or brothels. Although George complains about Lennie and says he could be like those other men, it is clear Lennie provides an excuse for George to have the loftier goal of their shared future plans. Being friends with Lennie sets George apart on a ranch where characters are judged harshly for marrying hastily or caring for an old pet. 
Both Lennie and George gain something from their shared friendship. Interestingly enough, Lennie gains the sense that he can be just one of the guys instead of being set apart, while George gains the reputation of being set apart because he is not like the other guys.