AMost of the changes actually take part in the second half of the film. At the beginning, there is an extension in Erebor. Thror teases Thranduil with white jewels he desires which help us understand his motivations later on. This way, the deep animosity between the Elves and Dwarves can be understood slightly better. Added dialogues with Gandalf in Rivendell are an improvement for understanding the story. For instance, the last of the 7 Great Rings of the Dwarves is mentioned. It will have a meaning at a latter point in the story - therefore, it is quite unclear why it was removed from the theatrical version. There is also a better exposition for the dwarf Bifur - for example, the axe stuck in his head is mentioned - an unusual device which was just taken for granted in the Theatrical version. However one has to admit that the changes in The Hobbit do not have the same impact or satisfaction for fans of the literary original as the ones for the Lord of the Rings saga. Apparently, there was already enough opportunities to adapt the (not too extensive) book material even for the theatrical version. An added song at the table is rather superfluous, a musical performance by the Great Goblin is even unnerving, and Bilbo's fascinated glance at the Ring is probably a bit too intense for his current character development. Some of the new scenes are rather a matter of taste; anyway, it is rather obvious that the filmmakers could not add as much valuable information to The Hobbit, as it was the case with Jackson's previous trilogy. Nonetheless a detailed comparison between both versions with pictures can be found here.