A lot of people are complaining about the adaptation of The Hobbit. I think that many of the changes are justifiable.
First, this is not a straight adaptation of The Hobbit and I don't think that most ticket-buyers want that. This is an adaptation of all of Tolkien's writings. He alluded to numerous events in The Hobbit and The Lord of the Rings. These are all shown.
In the book, Gandolf leaves the group half-way through. Dramatically, this was done to give Bilbo a chance to take over as the leader of the group. Without Gandolf, it is up to Bilbo to save the Dwarves from spiders and elves and to confront the dragon and eventually end the war.
When they meet Gandolf again he mentions that he was in an even bigger battle to the south. This was expanded in the Lord of the Rings where Gandolf discovered that the Necromancer they battled with was actually Sauron returned.
This sets things up for The Lord of the Rings which had not been thought of when The Hobbit was written.
The movies were made in reverse order so we know what is going to happen. A straight adaptation of The Hobbit would ignore this foreknowledge and disappoint the ticket-buying audience expecting a similar spectacle to the first set of movies.
Other points have been changed for dramatic impact or to fix plot holes. In the book, the dwarves have no plan for dealing with the dragon and there is no explanation of why they need a burglar. In the movie the idea is to recover the Arkenstone in order to rally the dwarf armies and kill the dragon.
Legolas wasn't invented until The Lord of the Rings but he was already hundreds of years old so he would have been there in The Hobbit. You can argue with the way he was inserted but he really needed to be there.
That's the good. The bad is in the execution. In The Lord of the Rings, most exterior shots were done outdoors with New Zeland acting as a special effect all by itself. In contrast, a lot of The Hobbit was shot in the studio and it shows. Murkwood went from an oppressive forest into a three-dimensional maze.
Many of the action scenes are over the top starting with the dwarves juggling Bilbo's dishes and continuing through the fights between the orcs and the elves and dwarves. These scenes would have more impact if they had been toned down. When a single elf can kill dozens of orcs single-handed without breaking a sweat there is no sense of danger. Compare that with Aragorn's fight against a single orc at the end of The Fellowship of the Ring. That was far more exciting.