Every guy may have his specific girl-next-door prototype, but in general, she's the girl whom you always admired from afar and were afraid to approach, fearing that any erotic projection toward her would ruin her image as a decent, pure and almost virginal womanly ideal.
She's (in your mind at least) untrodden ground; unspoiled by other men and so sweet-natured it almost frightens you to think of her in explicit sexual situations. Almost. Here's another example: Picture your sister's best friend from when you were kids. In your mind she'll always be that 10-year-old cutie who played with you in the park. Only, when you catch up with her 10 years later, she's still a cutie, but all grown up and someone you can trust and connect with on so many new levels. The intrigue grows.
Similarly, she could be a friend's cousin whom you met at family occasions over the years. Or even a new employee at work who is shy and unassuming, but still gives you that "she seems so nice, I'd love to talk to her" feeling.
Think of Renee Zellweger's character in Jerry Maguire