So let's start off this post by saying that I'm somewhat of a purist when it comes to making movies out of books, especially when it comes to what I've perceived of the person's character. I mean, I'm watching the movie because of the book, right? So I would reasonably expect it to be... true to it? Mostly?
That said, I liked the way they started off; the little prelude with Bilbo and Frodo makes sense, in a way, as it ties in the LOTR movies to this one - I suppose this was mainly to provide some familiarity for the movie fans who haven't yet read the book. The history of the dwarves was well put in the beginning for a movie, rather than in the random bits and pieces that trickled in through the book. And I just LOVED the dwarves' misty mountains song. It wasn't necessary. But it was nice.
My first major eh-what moment came when Bilbo runs up to the dwarves yelling for them to wait for him and that he'd signed the contract. Okay, so my memory of the Hobbit isn't as good as I'd like it to be, but if I'm not mistaken, Gandalf fairly chased him out of the house to join them. In fact, I don't think he ever actually agreed, but found himself being swept along anyway.
The next major bugbear I had, and this ran all the way through the movie, was the way they made Thorin hate everything so much. He hated the elves, he was nasty about Bilbo and he was overall a rather unlikeable person. Which annoys me, because whilst Thorin was aloof, he wasn't so overbearingly bitter. There was no major animosity against the elves in the beginning of The Hobbit, especially those in Rivendell - he only started to hate them because of their treatment of him in Mirkwood. In fact, there was some light-hearted ribbing between the elves and dwarves over their ancient rift, but nothing so... dark and ominous as in the movie.
I don't recall the stone giants appearing in The Hobbit, but whatever. And this led to another piece of character damage that I totally didn't agree with; Bilbo did not decide to leave the Company - in fact, he was the one who realised the door was opening and alerted them, which allowed Gandalf to escape. In the movie Gandalf wasn't with them because... he was holed up in a conference with Saruman, Galadriel and Elrond, whereby the three were against the dwarves' mission and Gandalf let them sneak out during their conference. I suppose the conference served its purpose - to explain the fall of Greenwood, and to bring up the Necromancer, though the Necromancer doesn't actually do anything in the Hobbit, so he could have been readily ignored. And whilst the dwarves were a little upset at missing Bilbo, they weren't as nasty about it as they were in the movie. In fact, they rather looked up to him after that because of his "feat" of escaping the goblins and sneaking past their lookout.
And then came the pointless battle between Thorin and Azog, who really shouldn't even be in the movie. The Wargs had come to meet the goblins, who were mourning the death of the Great Goblin, and which was why they were seeking revenge against the dwarves.
A friend rightly asked why the eagles didn't just drop them near the Lonely Mountains since they were being carried anyway, and that was where the filmmakers skipped the dialogue between Gandalf and the Lord of the Eagles, where the eagle-lord would not take them anywhere near dwelling places, and only because Gandalf had once helped them. Instead, they did the speaking to the butterfly thing again from out of LOTR.
I'm not really sure what to think about Radagast. I suppose he served a purpose in filling in some informational gaps that needed to be filled, but I didn't really like that they made him stupid-crazy eccentric. I rather imagined him distracted/absent-minded eccentric.
I suppose they needed the extra drama in the movie.
All that said, it was interesting as a movie, and I suppose some of the changes might possibly make sense when we see the next two instalments.
I still don't like what they did to Thorin.