There are many valid responses, his undying curiosity, his naive wonder and desire to understand how things worked, his lack of deference to authority and rebellious spirit. Another common answer may surprise you as it involves a quality rarely associated with physics or science in general for that matter.
Simplicity. Feynman’s genius lay in his ability to understand and present complex ideas in an intuitive and natural way. It is the reason Feynman remains an inspiration. He would exemplify this concept many times during his career, but never more so than in the development of his Feynman Diagrams. Simple line drawings that depicted complex sub-atomic interactions.
Read more at https://medium.com/@roblea_63049/richard-feynman-the-genius-of-simplicity-7f8610482414