First things first: The Hobbit: An Unexpected Journey is, essentially, fanfiction. You adapt a book to a film, some things are going to change - it's par for the course with moving between a medium where everything has to be described in turn to one where things can just exist side-by-side and be seen, between a medium where there's a narrative voice and one where that's usually kind of weird and intrusive. That's going to happen. It's still possible, though, to produce a pretty faithful adaptation of source material (just look at Watchmen).
Peter Jackson, Guillermo del Toro, Fran Walsh and Philippa Boyens have not done that. Their Hobbit has, well, made a few changes. It's developed a few characters and incidents, and added a few more; it's reshuffled quite a few plot elements; it's explicitly setting out as a prequel to The Lord of the Rings rather than 'that children's book that Rings was the sequel to'; it's recognisably the same story, but everything that Tolkien set off the page has moved onto the screen, executing a generic shift toward the epic. It's a film made by four people that love the book but want to seize control of it and do things with it, who love it for its potential rather than as a pristine and untouchable cultural artefact. It's exactly the sort of thing that good fanfiction does - but it's not one for Tolkien purists.
And, frankly, I think that's a good thing.
[What follows will, naturally, be spoileriffic. I wouldn't normally warn you about this, given that the book has been out since 1937 and all, but there's that much new stuff here that I think it's worth doing. If you care about spoilers do not click 'Read More'. You have been warned.]