I would say that one reason why Dr. King is considered great is because he understood the "fierce urgency of now," to use one of his great quotes. Dr. King understood that the pursuit of Civil Rights in American Society was not one that could wait. It had to be seized and owned, never to be relented. Dr. King was able to ensure that he was a product of his time, but also a force upon it. His ability to make the issue of Civil Rights for people of color an issue that all Americans could understand is another reason why he can be considered "great." Millions of Americans understood the fight for equality as not a "black" problem but rather one that struck at the very heart and essence of what it means to be American. Dr. King never missed an opportunity to argue that the principles of democratic self-rule made Civil Rights a logical extension of its essence. The "promissory note" as well as the calling out to Jeffersonian notions of the good life helped to bring this into reality.
I would say that another reason why he can be seen as "great" is because he was able to seize religion and transform it into a force of social change. Dr. King's vision of religion was not exclusionary but rather one that sought to bring more people into its fold. His use of religion was not demonizing nor was it isolating but rather a use where inclusion was practiced and preached. This helped to create the "brotherhood" and "unity" that was such a part of his beliefs and his life.