"The Hobbit" is a book by J.R.R. Tolkien, telling the story of Bilbo Baggins's (a hobbit) adventure with thirteen dwarves (not dwarfs) to reclaim the latter's homeland.
- Legolas's (a character of "The Lord of the Rings", another of Tolkien's works) presence in general;
- The addition of Tauriel (a she-elf) and the resulting romance between her and Kili;
- Gandalf's side quest;
- The mere appearance of orcs;
- Since they had to show the elves of Mirkwood, and since Legolas (who i think was from Mirkwood) was already known, and would probably already be alive (since the elves who appear are hundreds/thousands of years old), it would meant one less character to invent from scratch;
- For some reason any book that gets adapted into movie/s need to have some sort of love story. Regardless of how unromantic the original story may be. And as far as i can remember there are no women with a slightly important role in the book, so inventing was all that was left;
- In the book Gandalf comes and goes rather randomly, with the only explanation being that "he has to take care of some other business". So though the invented story makes little sense, I can understand why they placed it in the movies.
- Actually giving the king of Mirkwood a name. I can't remember whether his name is ever mentioned in any other book, but in "The Hobbit" he is only mentioned as the king of elves or Elvenking;
- Giving some sort of backstory to Bard the bowman. When he is introduced in the book he is simply the negativist bowman who descends from Girion, Lord of Dale.
- ORCS!!