I've heard it said that the Dresden Files don't really come into their won until the third book, and that they get better from there. It's hard to tell if that's really true, but I'm certainly getting much more engrossed in the world and characters as I get to know more about them. 
 The female characters, in particular, are getting much more interesting. I've complained before about the subtle (and not-so-subtle) sexism in the Dresden books. Well, Dresden is still a bit of an arse, but he's getting better. Though Murphy is largely absent in this book, Dresden does at least talk to her and tell her what's going on. It was always so frustrating in earlier books where he would bend himself completely out of shape to avoid telling her anything, just because he wanted to "protect her" (rather than, you know, helping her to protect herself by giving her the information she'd need to do so). 
 Susan, who features more prominently this time, is completely badass. In fact, I found it quite interesting that Dresden takes an almost completely passive role in this book as he encounters baddies who are just to big and strong for him. Over and over again, he is the one who is rescued - a few times by Susan. It's a great inversion and makes for a nice change. 
 The religion stuff was a bit silly (like Nicodemus referring to the book of "Revelations", plural), but it was easy enough to just go along with it. I did like how different Shiro and Sanya are, theologically, from Michael. When Michael was originally introduced, it was a bit of a groaner to have this perfect Knight of the Cross figure who ticked off all the stereotype checklist boxes. But Butcher adds some really interesting worldbuilding detail by having Sanya be an atheist and Shiro a sort of pantheist - yet all three are Knights of the Cross. The theology/rules are never explained (at least in this book), but it added a lot of nuance to what could have remained a very flat party. 
 I think that Molly was an another, less successful, attempt at this. Before we meet her, Michael's family is presented as Duggar-perfect, but she's clearly a rebel and isn't going along with the image Michael and Charity project. That being said, "rebellious teen" is hardly ground-breaking material. Neither, for that matter, is Wise Old Japanese Warrior. 
 Still, I found that Butcher really tried to invert a lot of stereotypes in this book - including ones that he himself had used previously. It was refreshing and interesting, and shows that he's getting more experimental in his writing - perhaps moving out from his comfort zone a little.