The key to their success is a warm,kind heart and, most important, self-acceptance. What is the usage of "most important" in this sentence. It means "most importantly" or " the most important key". Next question is : "The most important key" is only self-acceptance or both "a warm,kind heart and self-acceptance"?
It means that the most important , that is the main aspect, to the achievement of thier success is self-acceptance. A warm and kind heart is also part of the key to success, but it is not so important as self-acceptance.