One unattractive quality in a girl, or any human being on planet earth, is someone who hogs the spotlight. You know these people in your social circles: it's always about them, it's never about you, and it's never about the group. But a quiet girl doesn't need the boost to her ego to draw all of the attention out of the room or emotionally drain the people around her.
Self-centeredness can often result in over-talking, which is hardly attractive to guys who are looking for someone to spend their time (let alone their lives) with.
The way to treat someone is with respect and with balance. If you are hogging the conversation, it is hard to be on your side. But this is the kind of girl who knows, whether through nature or through nurture or through both, that people should always have the chance to speak their mind. Rather than seeing this as weakness, people should see this as a sign of respect.
Nobody deserves to be interrupted and nobody deserves to be talked over. She understands and recognizes both of these things, and she walks the walk (even if she doesn't loudly talk the talk).