I hear that phrase used all the time. But I never understood what it meant.
I always took it to mean a girl who was sweet, friendly and pretty, but not overly sexed up. A simple attractive girl, who probably wouldn’t make it as a model. YMMV.
An attractive young female who still appears somewhat “attainable” with an unsophisticated and enthusiastic personality. She dresses somewhat casually, no fancy expensive designer goods for her. Her tastes are not especially cultured, but she is by no means a troglodyte.
Contrast this with the “high maintenance” type or the “knockout”.
Just to give you a better idea of what a “girl next door” looks like, Jennifer Garner is the girl next door. Beautiful, wholesome, from humble beginnings.
Everything you ever wanted to know about the Girl Next Door type:
Cripes, according to that description it looks like I might be one. Can I get an age exemption? 
Oh, whew, I think I’m more the Academic Girl instead.
Ha, this is pretty much what I thought it meant. Thanks for the replies.
In my case, she was two years older, about 20 pounds heavier and possibly lesbian. She could also throw a baseball a lot further than I could.
Why don’t you just go next door and find out?
Married ex-cher impersonator.
There you go then!
She watches “Gilmore Girls”, and wants a baby. She’s waiting for you to call, but you never do [sub](you chickenshit)[/sub] so she gets knocked up by that slacker clerk she works with at Target, and has to stay in her parent’s house while going to Jr. College part time.
The “Girl Next Door” dates the “All-American Guy”.
Both are always white and live in the suburbs. And when they are missing or murdered, we’re all expected to mourn.
Clumsy bleeched, tanned blonde who goes to the local college, drives a new VW bug and keeps looking at herself in the mirror.
Ever seen Gilligan’s Island?
Mary Ann
Bingo!
See, all you had to do was ask! 
She’s sexy and either doesn’t know it or refuses to believe it. Which makes her all the sexier.
Mr. Blue Sky, how *you * doin’?
Having dated that type (twice)…that’s more frustrating than sexy.
I know, I mean, what’s the deal? It’s not they’re human beings or anything.