The Hobbit is the only Tolkien book I read so far, and I completely loved it. My problem with the movie is how they took elements from the book and changed it for the worse.
The biggest change is reoccurring villains. The book was a loose collection of various encounters with different obstacles, and everything tied into a huge climactic ending.
For the movie, there are two villains: Azog, a goblin leader (Thorin maimed him, he's back with a vengeance) and the Necromancer (he makes big spiders appear). They might be the same person, it wasn't entirely clear in the movie.
This is a huge change in tone! The book was all about Bilbo and how the adventure changed him as a person, a quest of self-discovery in a way. In the movie, it's a quest against evil again. All the hints about a "dark power" rising again and everything going downhill just detracted from the story of 13 dwarfs and a hobbit going on a treasure hunt.
And because of Azog, the group can't go for half an hour without fighting goblins on wargs. Luckily everyone seems to be a veteran fighter, and even Gandalf and Bilbo are much more aggressive. For example, instead of turning the lights off and escaping from the goblin fortress, Gandalf stuns everyone and challenges the goblins to fight.
Bilbo's combat skills are ridiculous. He says that he never used a sword and manages to defend himself against a goblin, then later kills a warg and a goblin.
And while I'm complaining about characters, the dwarfs were completely unmemorable. Partially this is Tolkien's fault, 13 protagonists don't work for a movie. At the end of the book, I had a pretty good overview about them as distinguished characters, but at the end of this movie? Well, Balin was the historian, Kili had a bow and arrow, Bombur was the fat one, there was one guy with a slingshot, and I don't remember the rest. The only dwarf with some characterization is Thorin Oakenshield.
Now, even though he was more fleshed out, he was still inappropriate as the group leader. In the book, he's really dignified and committed to the quest, it's pretty clear that he's in charge and responsible. In the movie, Gandalf takes a bit of that role. Thorin is more of a hothead, and much more brash and direct than his book counterpart. Also he hates elves with a passion, to create more conflict I assume. I especially disliked how he treated Bilbo: telling him that he doesn't belong to the group is just bound to create tension within the group. Insulting party members like a bully doesn't solve problems.
(The scene after they escaped from the goblin fortress was ridiculous, too. Everyone is worried where Bilbo could be, but Thorin just assumes that Bilbo went home, he didn't fit in the group anyways. It's not like he was last seen in a dark fortress with thousands of murderous goblins. Great leadership.)
Anyways, these were my main issues with the film. I could go on about the Necromancer sideplot that wasn't going anywhere or the stupid action scenes (why the hell did the film need two somebody-is-almost-falling-off-a-cliff scenes?), but the point is: I was expecting a good representation of the book, and this movie definitely isn't. Oh well, that's two movies that I don't have to spend money on.