First off, I have to say that I'm really pleased that this book has gotten a revamp, because I would probably never pick up the original edition. At least now, it's more appealing to teenagers, I suppose. That and they combined 2 books into one. :D Much more worth it. :D 
 The Fallen takes a very traditional approach to the Nephilim mythology. Unlike Cassandra Clare's where Nephilim are Shadowhunters and Becca Fitzpatrick's where fallen angels can kill their Nephilim vessel's descendent in order to gain mortality, Fallen's mythology deals with the traditional Nephilim mentioned in the bible. 
 Here, Nephilim are the offspring of mortal women and angels and they are offensive to God. The Powers are tasked with destroying all Nephilim. There's a prophecy involved but read it on your own. :D 
 There is quite a few mentions of God and His "views", and about the traditional Nephilim mythology but don't be fooled, it's not a Christian book. It's because of mythology involved. 
 There isn't much romance here so if you're looking for some YA romance, this isn't it. The main character isn't some suave, handsome, perfect main guy either so look elsewhere if you're looking for that. 
 Some might get turned off by the violence in the book but I think it really paces the story. It moves the story and it's tightly-paced. You should not get bored, but if you do, well, that's a shame. :D