Probably the only twist that those who didn't read the book saw coming before those who did.
Azog's hand, which had been chopped off by Thorin at the Battle of Azanulbizar, has been replaced with a metal fork. Azog is large, covered in scars, and most noticeably, is pale as snow. He is uninterested in Yazneg's excuses, and kills him for his failure. As the wargs tear Yazneg apart, Azog angrily demands the rest of the orcs send word out - he is placing a bounty on the Dwarf King's head.
The Book: No such scene occurs. Azog was killed at the Battle of Azanulbizar.
What difference does it make? The orc pack hunting Thorin's Company has already been discussed, but this time, we learn that they are headed by none other than Azog himself. This change is enormous; Peter Jackson is literally bringing characters to life that had explicitly and unambiguously died before the novel even started. Azog's death was never in question; his head was removed after the Battle of Azanulbizar. Peter Jackson is not only creating storylines of his own, but he's making changes on details that Tolkein had been very clear on.
My Opinion: I'll start with the good first: Azog is made far more interesting than he ever was in the books (or, should I say, the appendices). He was never called "the Defiler," and was never described as being any different from another orc, except for having a "huge head." The scars and pale skin make a very distinctive character, and as far as villainous titles go, "the Defiler" is as good as it gets. I'm irritated at his blue eyes, though. Why not just make him an albino? Not only would this explain why his skin is so pale, but it'd be the perfect opportunity to give him evil red eyes! It's a small detail, I know, but one I think that would have made a significant impact.
That said, this is probably the biggest and most controversial change Peter Jackson made for the movie. While I do understand the need to have some sort of villain created for the movie, like Lurtz from the first one, Peter Jackson had a couple of other options available to him. He could have invented a character, like he's done in previous movies, just to act as a sort of personification of some nameless book villains. Using the white warg, for example, and increasing the wargs pack's role a little bit would have served this role excellently. They could have began hunting Thorin's Company between the troll attack and Rivendell, and returned for the exciting conclusion. Or, if Peter Jackson wanted to make things more "personal" for Thorin, he could have used Bolg, Azog's son, who leads the goblin army in the Battle of the Five Armies. Bolg would have a personal vendetta against Thorin for killing his father (in Peter Jackson's adaptation), and his villainy would have lasted until the end of the new trilogy. Using Azog not only complicates the story of The Hobbit, it pointlessly makes changes to Tolkein's history of Middle Earth for the sake of a little more drama. This is a terrible change, made so much worse by some uncharacteristically bad CGI.
Plus it looks like he's having an orgasm when he kills this dude.