The Feynman lectures are classics. Although they are introductions to physics in the sense that they require essentially no physics or mathematics background, Feynman's lectures are beloved by some of the greatest physicists of our time. One reason for their popularity is that all of physics seems to be governed by a relatively small set of core concepts--conservation laws, thermodynamics, and wave theory come to mind--that Feynman identifies and stresses. For example, Feynman brilliantly devotes a lecture to conservation of energy very early in Volume I, long before he discusses Newton's equations. I believe that any person who deeply understands all of Volume I is a strong physicist!
Feynman manages to make deep understanding of important concepts simple, while never being condescending. He understood physics so well that he was able to identify the clearest and simplest ways of thinking about the subject. His tremendous insight allows the reader to have far less insight.
Another great feature of his lectures is that they are entertaining. Feynman shows his sense of humor often and presents in an enthusiastic manner. It is obvious that he enjoyed creating lectures and teaching. I sense that he got pleasure from the fact that he had such deep intuition about physics that he could teach in less traditional and more enlightening ways. Feynman's lectures are far different from other introductory physics textbooks, and in my opinion are far superior.
Feynman's lectures were given at Caltech, not Cornell as suggested previously.