Adaptation Distillation: Generally, the movie does a very nice job boiling down the novel into a good seventy minute movie, and some of the voicework is absolutely legendary (particularly Orson Bean as Bilbo, Brother Theodore as Gollum, and Richard Boone as Smaug). The only truly noticeable plot point left out is Beorn, who is totally absent, as well as the Arkenstone and the subplot involving it.
Adaptation Induced Plot Hole: The first meeting with the wood elves was left out... and then alluded to in the second meeting as "the wood elves had returned."
An Aesop: The film's theme song, "The Greatest Adventure".
The man who's a dreamer, and never takes leave Who thinks of a world that is just make-believe Will never know passion, will never know pain Who sits by the window will one day see rain
Dragon Hoard: Smaug sleeps on his treasure as if it were a bed.
Enemy Mine: Explicitly parodied at the end of the film.
Gandalf: Dread has come upon you all! An army of Goblins with claim to the treasure comes from the north! Behold! They ride upon wolves! Thorin: Oh, great Elf-king! My truest friend and ally! We must join forces against this common scourge! Thranduil: But... of course... o noble King under the Mountain. Your people are like brothers unto mine. Bard: And my Men and all their weapons are as one with yours. Together, we shall vanquish the foul foe! All: TOGETHER! Bilbo:(to himself) Thorin is correct, I simply do not understand war.
Large Ham: Quite a few of the voice actors bring the ham to their characters, Gollum's and Smaug's in particular.
Limited Animation / Special Effects Failure: The Battle of the Five Armies as seen from space. In the book, the scene cuts from Bilbo being knocked unconscious and missing the rest of the battle, to him waking up. The scene works here as an "and the battle continued for some time" passage, allowing us to pick up where Bilbo regains consciousness, unaware of what's happened or how things ended.
Roast 'em alive or stew 'em in a pot! Fry them! Boil them! Eat them hot! Bake and toast 'em. Fry and roast 'em 'Till beards blaze, and eyes glaze, Till hair swells and skins crack, Fat melts and bones black In cinders lie beneath the sky, SO THE DWARVES SHALL DIE!
A variation of Never Say "Die". There's a lot of talk about death, to avoid showing blood on-screen for the wee ones, slain foes spin away into Hammerspace like a sprite in a video game or a burst balloon.
This is a function of the various magic swords, and which death animation is used depends on the weapon. We see a few enemies just die at the battle in the end.
Most of them were musical adaptations of text-only songs from the book (which is not to suggest that they aren't thoroughly excellent). Some of these songs are at or near Ear Worm status, especially "Goblin Town" (and the Rivendell song in a way that is potentially more annoying).
The sword is sharp, the spear is long, the arrow swift, the gate is strong. The heart is bold that looks on gold... And dwarves no more shall suffer wrong. The King has come into his hall... Under the mountain dark and tall. The worm of dread is slain and dead, and ever so our foes shall fall!
Our Dragons Are Different: Although Smaug is based on European dragons who steal gold, he resembles an Eastern dragon and looks more cat-like than reptilian. This may have been influenced by the mainly Japanese animators.
In fact, Tolkien based his elves on the traditional myths about fairies, but by Tolkien's time, the term fairy had been corrupted to mean something unbearably twee, so Tolkien used the term elf instead, even though, as he well knew, traditional myths about elfs (yes, Tolkien also invented the use of "elves" and "dwarves," as opposed to "elfs" and "dwarfs," as the plurals of "elf" and "dwarf") were very different.
The film, however, compounds on this, because it actually makes use of the very vague descriptions Tolkien gave for the elves in The Hobbit specifically - while Elrond looks very noble, the wood elves are much closer to the original Scandinavian depictions of such creatures, being short, a bit ugly, and having blue skin. Which makes them very different from other depictions of elves in Tolkien's wake!
The So-Called Coward: When Bilbo expresses reluctance to join in the war between the dwarves, elves, and men, Thorin rebukes him and calls him a coward. As a slight Inversion, this is already after Bilbo has proved his bravery to the Company by rescuing them from the spiders and elves, and sneaking into Smaug's lair alone.