The "coming of age" theme in this play is a great point! Juliet's evolution is something many feminist critics have studied, and they've offered a number of insights about Shakespeare's point.
For example, if obedience was the preferred trait for women of that time, why does the heroine portray such aberrant behavior? Is there a hidden message, and if so, what might it be? What might Juliet's path from dutiful daughter to questioning, resourceful young woman suggest? And, what does it mean when Juliet seems to be the stronger one in the relationship?