I love Amy. I really do.
But she isn't as great as the other companions. I really think that the biggest thing that keeps the Doctor with her is immense guilt.
She has the attitude of Rose, in loving the adventure and danger, and looking after the Doctor. She loves Rory, big time. But you get the feeling that her helping others, other than Rory and the Doctor is because she thinks she should. It's not like Rose in series 2, episode 1, where the Doctor insists that Rose would care. If Amy didn't react, I don't really think it'd be the same kind of indicator. Rose, Martha, and Donna all really cared what happened to others. Amy gives the impression that she only cares because she cares what the Doctor thinks.
Which is why she needs Rory and Rory needs her. Alone, neither of them are as compelling as a companion, but he has the caring for everyone thing. He's brave for things that are the right thing to do, rather than headlong rushing into any danger he sees. He's perceptive with people who he doesn't necessarily have a strong connection with. Amy is more perceptive I think, for people who she really loves. Which isn't very many people. And she's right clever. And she will help others, but she appears to go out of her way to do it when the Doctor needs her to. Particularly in The Beast Below, in taking a chance to save the starwhale. When she didn't know the consequences with regards to her relationship with the Doctor, she chooses to forget and try to steer the Doctor away. But in The Rebel Flesh/The Almost People, she doesn't want to help Jennifer if it puts the rest of them in danger, and doesn't want Rory to go after her. I think Rose, Martha, and Donna would have, or at least put up a fuss about leaving her alone.
The best sum-up of her is the Doctor calling her "mad, impossible Amy Pond."
