That's such a deep question. I'm a bit concerned that everyone here is posting one or two sentences on the topic.
I look for someone who is strong; someone who is willing to stand up for what she believes in and not back down when she's told she's wrong. I think the sexiest thing in the world is an empowered woman.
The girls I tend to date are usually highly educated, and mildly sophisticated (I'm a country boy, so it doesn't take much to impress me sophistication-wise). They're usually fire signs, sometimes water, but rarely ever air or earth.
They also must be compassionate. They have to look at the world and see the horrible things that happen, and be able to empathize on a certain level with the countless victims of famine and poverty. Empathize doesn't imply
"Angelina Jolie," but they have to keep their hopes that the world doesn't destroy itself, and be passively working to reverse the effects of time and greed.
She should also be a woman of affection, as I am a pisces and incredibly affectionate. She should love to show her man that he's the only man in her world, and love to be treated the same. Monogamy and love are extremely important to me.
I could type about this all day, but I think you all get the picture. She needs to know when it's appropriate to "act like a lady" but also know that no matter what, I'll always support her so long as she's honest to others, and honest to herself.