She has personality, and because the show displays a wide range of emotions due to the simple fact that it's focused on her, she can seem perhaps even more human than some people we know.
She's a machine, she's strong, she's fast, she's smart, but she also makes mistakes, she gets scared, she gets self-conscious and she doubts herself.
We sympathies when character's struggle with the same things we've struggled with. She's a high-school girl, so she's worried about fitting in, about her appearance and how others see her, she's trying to make friends with a world that is amazed and afraid of her. A world that see's her as a tool for either great good or evil, but a tool nonetheless. You have a being with arguably sentience, in a world that see's her either as an object or less than human. There are many people on /co/ and 4chan as a whole who are still struggling with these things.
So yes, she's a machine, but she's just as human as any of us, and arguably it's her flaws that make her "attractive" to some people. Think about it, could you love a human that looked and acted like they had never experienced or could fathom a single failure, a single hardship which they couldn't insurmount?