Themes []
Commentary on isolationism []
Whereas An Unexpected Journey mostly illustrates and celebrates the Dwarves' patriotic yearning towards their homeland, and Thorin's need to do right by his people, The Desolation of Smaug plays up the isolationistic aspect of that worldview.
The reason Thorin so easily brushes aside Bard's arguments against the quest, is because his prime concern is the wellfare of the Dwarves, rather than the well-being of the People of Laketown. Bard, on his end, only really cares for the people of Laketown, or he'd be less vehement in his critique of the Dwarves.
A similar sentiment is expressed through Thranduil and Tauriel, with the Elvenking taking a more isolationistic stance, being content with staying locked underground while other lands suffer, and Tauriel not only being curious of the outside world but also wanting to activelly lend a helping hand to other lands and to members of other races. Her infatuation with Kili, a Dwarf, is therefore not unlike Thorin's friendship with Bilbo in that it transcends the isolation of the races.
The film posits that an isolationistic worldview will end up becoming narrower and narrower, to the point that those who uphold it will end up caring for the interest of no-one other than themselves. Thorin, for instance, ends of neglecting not just the safety of the people of Laketown, but also that of his kin (Kili) and his friend, Bilbo, all in the service of his own interest.
Leadership and amibition []
Bard is presented as a natural-born leader, as does Thorin. This is juxtaposed with the Master, who chooses to side with Thorin based on the reaction of the public, giving them what they want, instead of what they need.
This connects to the theme of ambition. The closer he gets to recovering what's he's lost, the more Thorin loses himself to the all-consuming desire to see the quest through. The opening flashback, presenting Thorin as initially reluctant to undertake the quest, makes it that much more ironic that he's willing to jeopardize not only the lives of the people of Laketown, but also those of Kili and Bilbo in order to see his endeavour through.
This feeds into the whole subject of doubting the moral high-ground of the quest. Elrond and Balin, in An Unexpected Journey, questioned the motives for the journey, but here the question is brought to the forefront. Several of the Dwarves recall their lives and families in the Blue Mountains: From Gloin's locket of his family and his gripes for having exhausted his money to fund the quest, to Kili's runestone. This culminates in the Dwarves appearantly giving up when the Hidden Door doesn't open. Although it is later insinuated that they were unwilling to stray far from the site, being that they returned within minutes at Bilbo's call.
Greed []
Thorin is slowly consumed by greed, to the point that he's willing to take a side with the clearly corrupt Master if it serves his interests in reclaiming his gold. At the sight of that gold, he becomes mesmerised and neurotic, and the potential of recovering the Arkenstone causes him to treat Bilbo with suspicion and threaten his life. He even takes to looking back at the gold, exposing himself to Smaug's fiery breath.
In this film, it is also made clear that the very hoarding of all the treasure, of a wealth that is beyond any measure and practical use, has been the cause of the Dwarves' donwfall. It not only brings Smaug upon the mountain, but also causes Thror and now Thorin to fall into greed. In that regard, the Dwarven monarchs are more like Smaug than they care to admit.
Narrative
The dispute over Girion's degree of success in fending off Smaug illustrates the different narratives held by different people, with the Dwarves having the view that Girion failed, whereas the descendants of Dale such as Bard comfort themselves with the narrative that he did manage to hit the dragon and - given another shot - will have killd him.