For me , Madonna's "big Secret" relies on her diligence .
She works hard as if she's still a nobody , people call her a bitch , diva or whatever but, whenever she commits herself to do something , she gives herself 100% and never goes like "I"m the Queen Of Pop" , "I'm the best selling female artist ever" , or any of those ***** !
I'm sure these things/titles/accomplishments are good to her ego but they NEVER get in the way when it comes to her *current* project(s).
She works hard and relentlessly !
Like she said during her NFL interview preceding the Superbowl , her father is the one to be blamed because he tought her that you have to work hard like day one for the rest of your life if you really want to become successful (or something in that vein

) and that's something she could never forget !