There's a theory that centaurs in this world actually are just as intelligent as they're usually portrayed, and their role as pets is a species-wide piece of Obfuscating Stupidity for some probably dark purpose.
Or, inversely, to not have to sustain themselves and instead be provided for by the humans (similarly to how cats started their relationship with humans).
There's also a rather popular theory that Mandy constantly orders Ella around so that Ella can learn to twist the orders and thus give her more freedom that way.
Considering that Lucinda is Lethally Stupid, as Fairest and Ogre Enchanted reveal, her Tears of Remorse and renouncing big magic come off as more suspect. Her gifts have caused people to commit suicide, or for them to be killed in civil wars. Is it that Lucinda renouncing big magic means she can't free Ella or is it that she's realized that she has too high of a body count to truly say sorry? Perhaps since Mandy pointed out that fixing big magic with another big magic spell makes consequences worse, Lucinda is unwilling to try.
Does Mandy know all of the terrible things that Lucinda has done such as inadvertently causing wars and suicides or just the blessings? If so, does that explain why she never forgives Lucinda, considering she's caused people's deaths with big magic? Or is it merely Mama Bear, that Mandy is furious about what Lucinda did to her charge Ella?
Harsher in Hindsight: Lucinda suffers a terrible Heel Realization about how she ruined people's lives with her curses. She becomes very apologetic for the rest of the book, while Mandy keeps reminding her about being a squirrel and an obedient child when it seems Lucinda is about to revert back to using big magic to help Ella. The later books Fairest and Ogre Enchanted show that before Lucinda learned this lesson, she had caused people's deaths in either wars or suicides thanks to the gift of a magic mirror, and nearly compelled one girl to eat others while turning her into an ogre. Yeah, she's sorry later on, but there is a lot of blood on her hands that one Apology Gift won't fix.
Iron Woobie: Ella. Cursed with absolute obedience, loses her mother, gains a terrible stepmother and stepsisters who abuse her and exploit her obedience, an apathetic father who's away all the time... yeah, this girl really needs a hug. However, her spirit, optimism, bravery, and intelligence help her soldier on.
In-universe, Mandy considers Lucinda cursing Ella to be this. She is angry that Lucinda doesn't understand the consequences of big magic and hurt a little girl under her care. Even when Lucinda renounces big magic and tries to help Ella as The Atoner, Mandy doesn't forgive her and keeps reminding her about the three months she spent as a squirrel.
Hattie can't hide being a jerk, so Ella already knows to hate her. Dame Olga seemed reasonable enough, however, before marrying Peter. Then she learns that Peter only married her for her money, after being "blessed" by Lucinda that they will love each other for eternity. She yells at him and then tries robbing her daughter in front of him by taking her purse and bracelet while they're riding in a carriage together. Peter then, for the only them in the novel, steps in and goes Papa Wolf, telling off Olga for treating his daughter that way and telling her to knock it off because Ella is now her daughter as well. When Hattie and Olive reveal that Ella is cursed to obey them, Olga takes great pleasure in turning Ella into a servant, punishing her for the crime of Peter being a lair.
Tear Jerker: Quite a few, the most notable being the death of Ella's mother in the beginning, and Ella pulling a Break His Heart to Save Him when Char confesses his love to her.
Unintentionally Sympathetic: It's hard not to pity Olive who seems to just be parroting her greedy mother and sister, especially since she's often portrayed as not being very bright and seems desperate for companionship.
The movie:
Ass Pull: So, Ella never once tried to give herself an order before? Maybe she needed an overwhelming reason to muster up the willpower to control herself, but even that is reaching.
Aiden McArdle is an elf in the land of magic who wanted to be a lawyer instead of an entertainer, for once. Then comes the Poirot episode Curtain, when he plays a man who never legally did anything wrong and can't be touched by the law, yet becomes a "magical" Manipulative Bastard who orders people to kill each other for him while playing with their emotions, thus escaping the law. Such bitter irony...
Ella's stepmother (even unaware of the latter's "gift" of obedience) orders the poor girl to break up her friendship with her old friend for being a "bad influence". Helpless, poor Ella has no choice but to answer the door and, well, you know the rest... The stunned look on her friend's face is just too much.
They Changed It, Now It Sucks!: Fans of the original novel who like the film are few and far between. Interestingly, Hathaway herself is a fan of the book who did like the changes done to the book's story.
They Wasted a Perfectly Good Character: In the original book, Mandy is Ella's Fairy Godmother who fights tooth and nail to help Ella survive both her stepmother's abuse and Lucinda's curse. In the movie, she is an incompetent house fairy who won't lift a finger to help Ella because she will lose her eternal youth and beauty (which she does not have in the original book, being a Cool Old Lady).