When Tolkien wrote The Lord of the Rings, he took all of the most interesting materials from Norse, Germanic, and Celtic mythology and created a conglomerate world of it; that's pretty well known. And when it came to the story itself he added in all the elements that would keep readers' attention - magic, elves, dwarves, disgusting monsters, pirates, ghosts, and the like. But at the center of his universe were four friends - harmless, skill-less friends. They were sent off to war as conscripts, they worked together and apart for a common good, they were honored above all for their accomplishments. In real life, they were Tolkien and his three comrades from school. Only he came home.
The Hobbit was a prequel, a device to set up the mythology of Middle Earth and to introduce the reader to some of the characters. There were some alterations from the book, but as an introduction to Middle Earth both movies have performed admirably. Like Lord of the Rings, the characters are vibrant, the themes are all present and the visual medium is used to enhance every aspect of the story. When a man like Orwell innovated camera angles, he was never able to do it with the subtlety Jackson has managed.
And like The Hobbit, Tolkien's ancestors may well have taken part in some of the wars and skirmishes that led up to World War I. For anyone who had been paying attention, the prequels to "The Great War" would have made any sane man shiver. In the movies, one is given the clear sense that the wizards know what is coming and are similarly worried.
Then there are the improvements on the book. Radegast is hilarious, and his character scenes rocked. The interplay between Gandalf and Galadriel could have never been attempted in written form. Nor could it have been used to make the many dwarves so interesting. (The introductory song in Bilbo's home comes to mind, and the fight scene as they escape the elves).
I suppose I should mention the odd addition of Tauriel to the storyline; she isn't a part of the original book. From what I have gathered so far she is a useful tool on several fronts. She helps to develop Kili's character. She makes Legolas more interesting. Potentially, she can be the bridge that allows him to make friends with Gimli in The Lord of the Rings trilogy. Personally, I like the addition.
As I like the expansion in the human village scene. The book is vague on what happens, and the eventual killer of the dragon is simply named without any background whatsoever. Not satisfying at all. With the elf woman, the lingering injury of Kili, and the human intrigues we are allowed a much better feel for him, giving the entire story a more rounded appearance.
I am generally of the opinion that if you want to give a book, comic, or show your own interpretation you should do what David Twohy did with Riddick and make your own. Then again, reinterpretations usually mean undermining or altogether forgetting about the original themes. The first X-Men series did that, and I hated it. The Hobbit does neither, it enhances a story that was already there, always paying respects to the original master. Peter Jackson can keep making trilogies for as long as he wants as far as I am concerned.