Selling the song

Whether you are singing for a recital, in an opera, or even in a rock band you have to be able to sell your songs. If singing were only about having an exceptional tone and wonderful pronunciation, millions of unknown singers would be able to be stars. What separates Placido Domingo, Michael Buble, and Barbara Streisand from the hundreds of singers that have more impressive voices? What factors contribute to an all around polished performance? How important is it to know what the text of the song means? Please share your thoughts about performing and what your considerations are when you "sell a song."