Just like women have to walk on eggshells around normal guys, normal guys are typically on eggshells around women, too. They don't want to say the wrong thing and piss a girl off, and they don't want to risk appearing too interested in sex or in women themselves and risk their harmless nice-guy status. Bad boys don't care about any of that. They laugh at girls getting pissed off at them – it's silly and cute when it happens – and they want women to understand they're interested in sex and in women themselves. Because of this, they're very open, honest, and confident in their dealings with women, and women feel this – and love it.
Nice guys tend to have unrealistic expectations of women: they often expect women to be these virginal, untouched perfect angels on best behavior at all times who always look beautiful and are responsible and mature. Nice guys also tend to expect a woman's full attention more often than not, and panic when they aren't getting it. Finally, they usually want a relationship right away, and will even chase for a relationship themselves, often before even having taken a girl to bed.
Bad boys don't have or do any of this. Their expectations and standards are far more realistic and much closer to the way women actually are – because they're based in real world experience rather than on television and movie portrayals of how women ought to be, like many nice guys' expectations are. They don't worry about not having a woman's full attention at all times, and maybe could even care less if they have it or not. And a relationship is, quite often, the last thing on their minds, instead letting women chase for the relationship – which women will tell you they don't like having to do, but you'll notice they value relationships with the men they have to chase for far more highly than the men they don't.
At the same time, nice guys, if they manage to be attractive in addition to being nice, have the potential to make good boyfriends or husbands, and women put a lot of pressure on themselves not to mess up with them and to portray their best selves at all times. With bad boys, there usually isn't much of a future a girl sees, so she can just relax and be real and honest with them without worrying about positioning herself as the ideal girlfriend or wife candidate.
I guess you could sum it up by saying it's a combination of the newness and unexpectedness of the bad boy with the comfort and ease he puts women in. He's exciting and refreshingly different, and at the same time he isn't heaping women with his expectations of them and getting upset if they don't meet them. Women can both find something new and just be themselves around him – and that's quite a rare thing for women to find in a man.
No wonder girls like bad boys.