Anne's development in her diary is extraordinary. She becomes more of a young adult through her trials in the Annex, growing both as a writer and a human being.
Anne begins her diary talking about the mundane things in her life—friends, boys, school, people she dislikes, her birthday party—with only a few mentions of Jewish persecution (having to wear the yellow star, curfews, not being allowed to have bikes). Once the family moves into the Annex, Anne becomes more thoughtful, even philosophical, worrying about more than just her own physical comforts.
Anne writes about the events outside more frequently, wondering about the outcome of the war and the presence of anti-semitism in Europe. She thinks about religion and love. She even thinks about women's place in the world due to sexist attitudes of the period, and how she wants to have both a family and a career. Most teenagers begin to become more independent in their mid-teens, thinking about topics and forming their own views on them, which Anne does a great deal in her diary.
Of course, she is still a teenager. She retains an uncharitable, even petty, view of her mother. She gets sullen when the adults treat her unfairly. But her growth is nevertheless impressive in such a short span of time.
In the end, Anne goes from being a perky child to a thoughtful, optimistic young woman who certainly had potential to become an engaging, humane writer had she only gotten to see the end of the war.