Jeet Kune Do as it survives today - if one wants to view it "reified" as a product, not a process - is what was left at the time of Bruce Lee's death. It is the result of the life-long martial art development process Lee went through. JKD in its later phases was heavily influenced by Western boxing and fencing (whereas the backbone concepts such as centerline, four gates, vertical punching, straight blast, "entering", and forward pressure come from Wing Chun). The result was that Lee stopped using some of the Wing Chun stances he had learned in favor of what he claimed were more fluid, flexible Western fencing and boxing stances. The claim is that allowed him to "flow", not to be stuck in stances, a positioning that Lee believed was a feature of some of traditional Wing Chun that he dismissed as the "classic mess". For instance, instead of using footwork to position the body for maximum fighting position vis-a-vis the opponent, JKD uses flowing boxing "entries" that do not require "bridges" from Wing Chun.
Dan Inosanto, who was to be Bruce Lee's successor in JKD, once said that originally, Bruce Lee wanted to create the "ultimate fighting form", but later in the development of JKD, he wanted to use the art for personal development, not just to become a better fighter.
JKD not only advocates the combination of aspects of different styles, it also has to change many of those aspects that it adopts to suit the abilities of the practitioner. Additionally, Jeet Kune Do advocates that any practitioner be allowed to interpret techniques for themselves, and change them for their own purposes. For example, Lee almost always chose to put his power hand in the "lead," with his weaker hand back, therefore he almost always used the right hand stance of Wing Chun in JKD and discarded the left hand and center stance, whereas traditional Wing Chun and other Chinese martial arts train their practitioners to be ambidexterous.
Lee emphasized what he believed to be the combat effectiveness of JKD, and did not stress the memorization of solo training forms the way that most traditional styles do in their beginning level training. While practicing Western wrestling moves, Lee was once pinned by a more skillful opponent, who asked what Lee would do if he found himself in the situation in a real fight. Lee replied, "Well, I'd bite you, of course." The JKD theory is that a fighter should do whatever is necessary to defend him or herself, irrespective of where the techniques used come from. Lee's goal in JKD was to break down what he claimed were limiting factors in the training of the traditional styles, and seek a fighting doctrine which he believed could only be found within the event of a fight. JKD is nowadays seen as the first of the modern spate of mixed martial arts.

JKD followers claim that it is not a fighting style so much as a fighting philosophy. What JKD practitioners describe as the weakness of traditional martial arts is their rote memorization of techniques (Lee compared doing forms without an opponent to dynamically try them out on to learning to swim by doing the breaststroke on land). They claim that these memorized movements will not be of help in an actual street fight. JKD does not make one a good fighter, they claim, it makes one a better fighter.
Bruce Lee's comments and methods were seen as controversial by many in his time, and still are by many today. Many teachers from traditional schools disagree with his opinions on these issues, especially seeing what Lee described as their lack of strategic flexibility due to "rote" teaching methods to be a misunderstanding on Lee's part. Most, if not all, traditional martial arts teachers say "fluid" strategy is a feature of martial training that is indeed addressed in the curricula of most traditional styles at advanced levels, when the students are ready. The schools Lee criticized tend to see their initial conservatism as a safety feature; a legacy of practical experience passed down from generation to generation, said to ensure that their students are thoroughly prepared for advanced martial training, skipping nothing and developing intangibles such as good character, patience and discipline. The hierarchy of the traditional schools is said by this reasoning to provide a level playing field for all students by instilling respect and care for one's seniors, peers and juniors, so that everyone, not just the physically gifted, has an opportunity to benefit from the training provided in a martial art school.
The notion of cross-training in Jeet Kune Do is similar to the practice of mixed martial arts in modern times -- Lee has been considered by UFC president Dana White as the "father of mixed martial arts"[1].
http://en.wikipedia.org/wiki/Jeet_Kune_Do