Willow becomes a more powerful witch then anyone expects her to be. She also, controversially, discovers that she is a lesbian, and falls in love with her fellow witch Tara. Willow went from being a somewhat weak and gentle young girl who was powerless against the people she saw as better than her to being a witch with power stronger than anyone believed that she was capable of. She didn't accept herself, but she found a strong group of friends who always accepted her (with the exception of Cordelia) and helped her do the same. The Scooby gang has a hard time with the idea that one of their best friends is a lesbian, but they love her so they accept her.
Her change from being a weak and helpless individual to being one of the strongest characters on the show is so inspiring because it shows girls that it is okay to be who you are, no matter how nerdy you are or who you love. Once you find your group of true friends, they will love you no matter what and give you the power to accept yourself. Her change was by far the most drastic and the most incredible.