Many parents have this question, especially when available Montessori options are tuition-based. The American Montessori Society (AMS) has put together a nice answer on their blog here:
The attributes listed above are going to be of primary importance to a credentialed Montessori program. They are at the heart of the program we're developing here at Mariposa. Most importantly, they are at the heart of a love of learning. Students who are recognized as individuals, allowed to proceed at their own pace and warmly welcomed into a diverse community of learners will thrive.
Sir Ken Robinson, one of my inspirations, put it this way: Education needs to be personalized rather than standardized and we should be nurturing the spirit as well as the mind.