From my hastily scribbled notes, the top of the cast list went something like this:
1st. Ian McKellan (Gandalf)
2nd. Martin Freeman (Bilbo)
3rd. Richard Armitage (Thorin)
4th. Benedict Cumberbatch (Smaug and Sauron)
5th. Evangeline Lilly (Tauriel)*
6th Lee Pace (Thranduil the Elvenking)
8th. Stephen Fry (Master of Lake-Town)
My first response: it just feels wrong, somehow, to not have the actor playing Bilbo, THE Hobbit, get top billing in a movie named THE HOBBIT.
On the other hand, I can see the argument that if you've got Sir Ian McKellan playing a lead role in your movie, that's a Good Thing and you want to put that fact front and center.
So I can see both sides of this, but it still struck me as a bit odd.
The second point that arises from this cast list sequence is how important the new characters are to this second film.** Most of the returning characters from the LotR films who helped anchor the first Hobbit movie to Jackson's earlier trilogy (Elrond, Galadriel, Saruman, Gollum) don't reappear in this second movie. Gandalf is really the only such figure to play a prominent role here. And of the important characters introduced in AN UNEXPECTED JOURNEY (essentially the fifteen members of Thorin and Company plus Radagast and Azog), all except Gandalf, Bilbo, and Thorin play smaller roles in this one, the second-tier roles essentially being reduced to make room for the new characters introduced in this second installment. The high quality of some of the performances of characters introduced in Jackson's second LotR film (Theoden, Eowyn, Gollum) having been a major factor in that film's success, I thought it'd be good to take a look at this second HOBBIT film's equivalences.
*Thranduil (the Elvenking)
We'd glimpsed this character in the prologue/flashback at the beginning of AN UNEXPECTED JOURNEY, but we get to see much more of him now (plus, he gets to have more than one expression). First off, I was vastly relieved to find he's not another Denethor. Denethor was portrayed as a malevolent loon in Jackson's RETURN; by contrast, in the Elvenking we have an unsympathetic but well-motivated character, and smart to boot (he pretty much figures out Thorin's plan from the get-go). He's closer to the Elrond of the original movie trilogy (the Rivendell scenes in the first HOBBIT film having featured a kinder, gentler Elrond, more like the character Tolkien had described). He's an interesting character in that what he offers sounds reasonable on the surface, so that Thorin seems churlish to reject it with insults. But we're shown clear signs that Thranduil can't be trusted (as when he murders a prisoner he'd promised to release), signaling that Thorin was probably right not to make a deal with him; it wd be entirely in character for this Elvenking to renege on any agreement made "between kings" on the basis that Thorin is not, yet, a king. It's a particularly bad sign that in a sentence or two Thranduil echoes the Goblin King, right down to phrasing.
--the odd little scene in which Thranduil lost control and part of his face melted away puzzled me a bit, but I think Janice got it right when she said this was elven glamour that masked the Elvenking's true face slipping for a moment under stress. That makes sense, given the glamours we're shown in the book of elven feasting among the trees: what we see in Mirkwood and the wood-elves' realm isn't necessarily reality. The scars also help lend weight that he knows whereof he speaks when it comes to just how dangerous dragons are.
The most important character contending for Not-Appearing-in-the-Book is clearly Tauriel. I don't suppose her creation and insertion is a bigger departure than Elves-at-Helms-Deep, or for that matter Arwen's presentation as Arwen, Warrior Princess in the FELLOWSHIP film, but it's still the point at which purists are allowed to have their qualms. That said, if they were going to invent such a character, at least they did a pretty good job. For one thing, they gave her a reasonable outfit, which puts them ahead of most online fantasy games, many an rpg cover, and quite a significant percentage of fantasy book covers as well.
As a purist, I naturally prefer the film to follow Tolkien. But I find some departures bother me more than others. In particular, I think I'd prefer a new character be invented to fill some plot function in the movie than that a genuine Tolkien character be distorted beyond recognition (as with the TWO TOWERS' Faramir). Thus the introduction of a not-quite-as-good-as-Legolas elf-warrior doesn't particularly bother me, though I cd have done without the romantic subplot.
After having been a bit put off by the character design, I found I rather like the movie's Beorn, though I thought this section was too brief. Maybe we'll see more here on the extended edition.
Here's a major character who looks rather like Inigo Montoya but is given a personality more like a combat-adverse Strider. That makes a certain amount of sense, since Bard and Aragorn are both rightful kings of fallen kingdoms. His sudden change from helping the dwarves to publicly hindering them was too abrupt for me, though I understood the logic of it. On the whole, good enough, though the jury's still out, depending on how well the character comes through in the third and final movie.
Here's an odd case of Azog being given Bolg's role (in the book, where he's the commander of the goblin/warg army) while Bolg is given what had been Azog's role in the first movie (in charge of the pursuit and persecution of the Dwarvenking). Adequate, but not quite up to the standard of the orc-leaders in the LotR Jackson film trilogy.
Enormously impressive, though I don't know why they altered Tolkien's character design to make Smaug look clumsier (Tolkien's Smaug doesn't have his forelegs attached to the wings, and thus was more sinuous and graceful). Much of Tolkien's original dialogue between Bilbo and Smaug made it in the famous talking-to-the-dragon scene, which is altogether a Good Thing. Cumberbatch did a great job with the voice. The whole running combat between the dragon and the dwarves went on much too long, but at least the Smaug scenes captured the dragon's power -- this is not a some garden-variety dime-a-dozen wormling but a truly impressive wyrm.
The Master of Lake Town (and sidekick)
What a wasted opportunity: the great Stephen Fry given little to do and doing little with it. His sidekick does his best Uriah Heep, but that best is not good enough. Pity.
For a lot of movie fans of the LotR film trilogy, the reappearance of Legolas must mark a high spot in the HOBBIT films so far. For me, he almost took over the wood-elves, barrel-dwarves, and (new) combat-in-Laketown sequences, but not in a good way. In this film Tauriel serves as a sort of second-tier Legolas; I hope that's reversed in the next one, so that she's given a larger part than his. We'll see.
And, of course, we've yet to learn what new characters may yet debut in the third and final film.
current reading: TREE BY TOLKIEN by Colin Wilson 
THE DESOLATIONS OF SMAUG VISUAL COMPANION by Jude Fisher 
*my note for 5th place isn't particularly legible, what with the dark and 3Dglasses and haste and all, so it's possible that instead of "Lilly" I might have instead written down "Legolas". My memory says that Orlando Bloom (Legolas) came in the end-cap position on the top-cast list, but memory can be deceiving; this'll have to wait until another time to know for sure.
**as if to emphasize this point, the cover of the movie tie-in book THE HOBBIT: THE DESOLATION OF SMAUG VISUAL COMPANION by Jude Fisher has Tauriel, Bard, and Bilbo on the front cover (along with some barrel-dwarves), while the back cover shows Legolas.
CORRECTION: Updated Th.12/26-13 to correct Inigo Montoya's name, as per the comment by Robert. Thanks for the correct, Rbt.