It's nice to be in control of your life, right? In Ella Enchanted, however, there are about a zillion ways that people can exert power over another's life. People gain (or lose) power through magic, heredity, money, duty, and love. The power to decide one's own fate is held up as something to strive for, even while the ties that constrain characters' choices are on display. To recap: power is awesome. But do any of the characters in the book hold absolute power? And would that even be a good thing?
Ella's noble birth puts her in a privileged enough position that the spell she's under does not completely disempower her.
The power to disobey is part of what makes us human.