She's blonde, young, thin, white, tall, attractive, American, and has blue eyes -- not sure about the last one, probably true.
She also seems to bond well with her fans, isn't a big bitch always doing drugs and partying with photos of her drunk off her ass lying in the street, comes off incredibly nice, actually. No confirmed nudes yet either.
Her music is catchy, she mostly writes her own songs and plays at least two instruments, isn't a one hit wonder with five albums now and the latest broke sales records at a time of declining music sales. She should be worth over $200 million now at 25.
No, she isn't the best singer. Her music doesn't have as much depth as the other stuff you referred to, I think, but it's still very personal that, for some reason, kids and adults can relate to in one way or another.