Dame Margot Fonteyn becoming the prima ballerina at Sadler's Wells Ballet. As the art form grew, ballet became more and more susceptible to other influences, most notably modern dance. Over the years, the cross-pollination of the two forms has become so thorough that some new works are difficult to unequivocally categorize as one or the other. Many ballet companies have incorporated works by modern masters such as Twyla Tharp and Paul Taylor that have much less to do with ballet than with modern dance or even jazz dance. It can be classical or contemporary, narrative or abstract, tragic or comic, pretty or ugly.
Some ballets are showcases for virtuosity, while others tug at the heartstrings.
And shoes are no longer a clue. Ballet can be danced in any number of shoe styles or even barefoot. Now, the actual categorization is almost irrelevant. Dance is more eclectic, more catholic, and consequently often more vital than ever. But for the purpose of trying to understand and appreciate the form, the most important hallmark of ballet is the technique.