So bard in the books was the captain of the guard in laketown. He is a decendant of human ings of the north. Not some smuggler like in the movie. I don't know why they changed this for the movie as it seems very strange and seems like it might actually cause a potential problem in the future. Because in the book, Bard actually kills Smaug when the thrush that Bilbo sends to him to tell him of the missing scale on Smaug's belly. Bard is in a position of power in the town to negotiate with Thorin for money for the town when Thorin goes all greed master in the book after Smaug dies. Which is why Thorin basically tries to start a war over keeping the treasure they just got back from the Dragon without sharing it despite promises earlier in the book to do so.
I do like the extra bits they threw in with Gandalf to fill in the return of Sauron. That whole bit of scenery at Dul Guldor (sp?). This was not in the book at all as all the references to Sauron was not even really in the Hobbit. Mainly because Tolkien hadn't even realy thought about writing the Lord of the Rings trilogy and was actually cajoled into writing the sequels later. Still, I really liked that bit as it was a nice tie in with the later movies and explains part of the hole of Gandalf leaving the party in the book a bit more. Of course in the later revisions of the Hobbit a bit more of the approaching greater evil was added, but still not a direct reference to Sauron nor the ring Wights.
The interaction between Bilbo and Smaug in the movie was quite a bit different than that of the book. However, I LIKED the difference. This is because the book goes into a bit more detailed explanation of the powers of the one ring, of basically being able to dampen ALL forms of detection for the wearer. So Bilbo while wearing the ring can't be seen, smelled, or heard unless he wants to be. Of course the ring doesn't stop the emanation of evil that it exudes. So beings like Smaug can detect that. In the book, Smaug doesn't even really notice that Bilbo was there until Bilbo takes a gold cup. Smaug notices the missing gold cup despite the vast treasure, which shows how great his mind is at cataloguing the treaure. Anyhow, the by play between Bilbo and Smaug was done entirely with Bilbo being invisible. Since the movie doesn't want to do a hard narration of why Smaug can't detect Bilbo but still vaguely senses him nearby, they just have Smaug not really attack due to his own sense of Ego and not worried about something so small as Bilbo. It plays into the character of Smaug very nicely from the book. So I liked that adaptation. Of course the fight between the dwarves and Smaug doesn't happen in the book. Oh well a little action for a bit of a dull moment I suppose that didn't actually change the basics of the story.
As I said previously, I do like the inclusion of Azog the White Orc into the story in the movie. He was not in the book, but he adds a bit of face for the audience to dislike. He becomes something for the audience to identify with instead of a bunch of nameless orcs that were after the band for no real reason in the original book. Again a plot hole from the book that I feel the movie made up for nicely. It also gives a slightly better reason why the Orcs with the Wargs show up to fight at the end of the book in the Battle of the Five armies (humans, elves, dwarves, orcs, and wargs). The orcs follow Azog, and Azog follows Sauron. Sauron doesn't want to see the dwarven kingdom rebuilt to help fight against him later when he gets enough power to enter the material realm again.
Again overall, most of the points of the movie stayed very true to the book. There were obvious differences. Most of which I liked. Some of which I didn't, such as changing who Bard is fundamentally.