Bruce Lee & Wing Chun
Bruce Lee's fighting system is not modified Wing Chun. This is a big misconception to all except his Wing Chun friends and those of his first students. Bruce's and Wing Chun's definition of modification are different then that perceived by most of his students and the Western mind.
Wing Chun set's itself up for "modification". It's meant to grow and develop with the student and become an "expression" of them. Wing Chun allows change for the sake of adaptability and survival. Those that know enough about Wing Chun also know that there really aren't any such things as "Wing Chun techniques". These movements are only an expression of its concepts. If along the way a student finds a better way to express Wing Chun concepts while adhering to proper body structure and principles, they can modify the techniques to as they see fit. Because of his many personal experiences and lack of knowledge in certain areas of Wing Chun (he only knew the first two "instructional" forms, and a limited number of the wooden dummy forms), he came up with his "personal" solutions by using his own ideas and guidance mixed with those he had already learned, further developing his own "personal" Wing Chun tools. This personal Wing Chun he called Jun Fan to his early students. Jun Fan was Bruce's Chinese name.
He might have developed different "solutions" had he stayed in Hong Kong to continue his Wing Chun study. Instead, he was thrust into a world of many different styles and systems. He dealt with them and allowed his personal style to grow based on the guidance of his Wing Chun knowledge. No different a process than anyone else does in Wing Chun.
When he first started teaching, he taught formal Wing Chun. As his knowledge and personal style grew, he felt it was enough of his own "style" to label it Jun Fan Gung Fu, as was stated earlier. Everything he did was still in a Wing Chun frame, because Wing Chun is still that frame of changeless change. You are constantly changing and improving and "modifying" your ideas to form your expression. He still had his Wing Chun ideas and attributes, no matter what he applied them to. When he did the straight punch, he started from the middle with the elbows down, which is done in Wing Chun. When he used a Northern Shaolin side kick, he still issued power with a stomp as a Wing Chun practitioner does. When Bruce did Kali sticks or nunchakus, he maintained his upright Wing Chun posture.
As Bruce went on, he applied the ideas he learned in Wing Chun (Economy of motion, Non telegraphic, balance, among others) to all that he came across and picked up.
What Bruce meant when he said he saw limitations in Wing Chun was that he saw limitations in some of the physical techniques, and the way they were performed. He then "adapted" and "modified" them to the situation at hand which is what Wing Chun allows for in the first place. He also added different techniques to his Wing Chun one's, for solutions to situations that he felt the Wing Chun techniques weren't designed for (alot of times, it was because he had lacked the training to correctly apply the Wing Chun techniques. Both solutions would be correct though. It's what gets the job done that counts). When he added these new techniques or sometimes theory's, he made sure they fit with the guidances he had already established in Wing Chun. So if a new technique landed, but say he was too far off balance, he threw it out. If the new solution he came up with involved too many movements or did not follow a simple enough path, then it was thrown out. This is what is meant by "absorbing what is useful". The only thing that was "modified" were the actual physical Wing Chun techniques. Again, this is what Wing Chun is really all about.
The above ideas are an adaptation of ideas taken from http://www.wingchun.com/JKD.html