It also has to be mentioned that one of the most important elements to the movie is the soundtrack. It's full of upbeat, ineffably classy ragtime piano music that better than anything defines the je ne sais quoi charm of the movie. The music is mostly composed by Scott Joplin and performed on the Official Soundtrack by Marvin Hamlisch. The centerpiece, masterpiece, and theme song of the movie is an arrangement based around The Entertainer.
It should also be noted that this film won seven Academy Awards, including best picture of 1973.
Thanks to halspal for pointing out an obvious error in the original writeup.