See my review of the first one here. I will not be naming returning cast members from the first film.)
How closely these films have stuck to Tolkien's text has been a matter of debate since the first one came out. The Lord of the Rings films had their departures from Tolkien's text, but were largely faithful to the story you could find in the book.
The Hobbit has proved a different matter: The Lord of the Rings is a weightier book than its predecessor, which is often published split into three books, each of which is still longer than The Hobbit. Having once penned a fan script for the classic Tolkien book myself, I'm very aware of how the story in the book can be expanded on.
The big issue facing Jackson was that he'd made The Lord of the Rings before The Hobbit. Of all the characters that would be returning in the prequel trilogy, the most problematic was Gandalf, played by Ian McKellen. Gandalf leaves the company shortly after the events of the first movie, and in the book, he had been set up to be a mysterious character. But now that The Lord of the Rings had portrayed Gandalf as a warm, kind man, having him suddenly turn mysterious and vanish from the second film almost entirely would be confusing to audiences. The answer to how to respect both the character's portrayal and Tolkien's text was to actually show what Gandalf went to do, which Tolkien revealed was actually quite important to setting up the events in The Lord of the Rings. (This expansion had begun in the first film.)
In contrast to the first film, the second movie begins with many action-filled pieces. I expect (and hope) that The Desolation of Smaug will be receiving a more substantial extended edition as the beginning of the movie flowed very quickly through the company lodging with Beorn (Mikael Persbrandt) and Mirkwood, cutting out a few points from the book that I would like to see depicted by the cast and Jackson.
The movie begins to slow down as the dwarves are captured by the Wood-elves, including captain Tauriel (Evangeline Lily) and prince Legolas (Orlando Bloom, reprising his role from The Lord of the Rings). When Thorin decides not to accept help from King Thranduil (Lee Pace), the dwarves are kept imprisoned. Kili strikes up a romance with Tauriel (a scene in An Unexpected Journey's extended edition featured him eying elves in Rivendell), probably one of the more non-Tolkien-like expansions of the story for the film. Bilbo soon helps everyone escape in barrels, which are attacked by Bolg and his orcs, leading to another action scene in which Tauriel, Legolas, and surprisingly the heavy dwarf Bombur get to be the big heroes.
Gandalf goes to the High Fells and finds that the Ringwraiths have been summoned from their tombs, and then he heads to Dol Guldur to confront the Necromancer, but is captured when he discovers that Sauron is based there.
Finding the secret side door, Bilbo enters Erebor to find the Arkenstone, but instead awakes Smaug (Benedict Cumberbatch through voice and motion capture, who also portrays Sauron). In an extension from the book, the dwarves enter the Mountain to attempt to fight Smaug on their own by tricking him into lighting the forges, but Smaug is still too strong and decides to attack Esgaroth, ending the film.
While bigger Tolkien nuts can list more reasons why they don't like Kili and Tauriel falling in love, to me, it just seemed like an unnecessary touch. It could have been more effective if all it was was Kili trying to make her let them go. Perhaps something more will be done in the final film. Some fans have also pointed out the compression of the timeline in Jackson's films, but we must remember that this is a visual medium Jackson works in, and it's important to show and not tell.
Leading on from that, it's hard to say whether or not some of these changes and additions are actually going to work out until we see the next film. Unlike other trilogies, Jackson's Middle-Earth trilogies were envisioned as whole pieces. However, I quite enjoyed this one, even with some of its stranger expansions and changes. Smaug is the best movie dragon I've ever seen, and the cast and crew continue to deliver.
It's important to remember that this isn't J.R.R. Tolkien's The Hobbit, but Peter Jackson's. Whatever changes Jackson makes will be wholly his own, while Tolkien's original book will still be around, unaltered for current and future readers to enjoy for what it is.
All the same, I quite enjoyed The Desolation of Smaug and really look forward to December 2014 when the story will wrap up in There and Back Again!