Incredibly bad casting (with one exception)
This movie wasn't bad, but it was done in by the casting. First off, because a character is 15, doesn't mean you have to hire a 15-year-old to play her. (That can be like hiring someone who is actually drunk to play a drunk). Dakota Fanning is a child actor who hasn't quite shown she can handle a more serious, more adult role.
Also, the idea of her being the "sexy" front of the group is pretty hard to buy when you cast an awkward teen like Fanning against older and more attractive actresses like Kristen Stewart and Scout-Taylor Compton. Cherie Currie was prettier than Joan Jett, but Jett had more charisma and definitely more talent. Stewart manages to both be prettier than the real Joan Jett, but also much, MUCH less charismatic. The best young ACTRESS here meanwhile is Scout-Taylor Compton, but she hardly has a role.
The one actor who was great though is Michael Shannon. I don't know why he hasn't gotten nominated for an Oscar yet. But they should have used Jennifer Lawrence for Cherie Currie and pretty much anybody BUT Stewart for Joan Jett.