Take a look at the movie poster on the main page, with Bilbo Baggins framed by the door of Bag End. Not only is the door certainly meant to be a visual metaphor for the One Ring - but the shadow cast by the light and the dark of his silhouette makes up the eye of Sauron.
The final image of the film is Smaug's eye opening. Looks a bit like the eye of Sauron as well (yellow, red rimmed)
In the film, Thrór is described as succumbing to a "sickness" over his longing for gold, and it is stated that his desire for more and more wealth is what drew Smaug to Erebor. Now, consider the back story to LOTR: The Seven Rings of the Dwarves were captured and corrupted by Sauron, who then presented them to the kings (Dúrin's Folk dispute Sauron ever had his ring, of course, but it's left ambiguous by Tolkien). However, Sauron discovered that because the dwarves were specifically made to be resistant to evil by Aulë, he couldn't directly control or corrupt them the way he could use the Nine against men (another bit of brilliance in the film version of The Fellowship of the Ring: this is the one thing that makes it plausible if anyone could try to break the Ring by himself, it would be Gimli). What did Sauron do instead? He twisted the Rings themselves so that while the dwarves could indeed use them to build their fortunes, that it would also lead to the destruction of their kingdoms. Thrór possessed the Ring of Dúrin. His madness for gold, which led him to accumulate such a large hoard it drew Smaug's attention, is the result of his Ring's corruption. And it gets even better from there. It's implied that, since Thrór's son was mysteriously absent at the time of his death, Thorin inherited Thrór's Ring of Power in his father's stead. This Ring could have been used to twist Thorin's darker characteristics against him - his anger and thirst for vengeance at the loss of his home and the dwindling of his family are the easiest targets for him. Now, when Thorin gets his danger face on and attacks Azog from out of the burning tree near the end of the film, listen carefully to the music that plays. It's the Nazgûl leitmotif from the LOTR films. And we see that Thorin's rage drives him to make an unwise attack that surely would have got him killed if Bilbo hadn't interfered. The logic follows that Sauron is using Thorin's Ring to manipulate him into getting himself killed - thus eliminating a powerful ally who might have been of great help to the good guys in the coming War of the Ring. And, if you've read the book, you can see that this tactic works. Not only is Thorin going to be killed, but Fíli and Kíli, his nephews and the only heirs of his line, die defending him. Let that sink in a bit. Sauron truly earned his Magnificent Bastard stripes.
An Unexpected Journey. One of the dwarfs finds an arrow and disgustedly declares that it's Elven. This parallels the similar scene from LOTR, save that that arrow was Goblin in origin. Now, obviously, the racial tensions between dwarfs and elves cool down, to the point of genuine friendship in some cases, in the time-frame of LOTR. If we push this parallel, could it be the filmmakers' way of saying, "Hey, even the Orcs could be welcomed among the beings of Middle-earth some day"? There is basis on Tolkien canon, given that Tolkien wavered on whether the orcs were really Always Chaotic Evil.
Gollum, while still being creepy, is willing to play a game with Bilbo and seems like less of a threat. This contrasts the darker aspects of the character seen in the original trilogy. The reason? In LOTR, Gollum has been separated from his Precious for sixty years, on top of his Ring-induced craziness and prolonged torture at Sauron's hands for information about its whereabouts. He's also quite happy to have a 'playmate', after five hundred years with only himself to talk to, particularly one of his own kind who knows riddles. In the movie, he may have had a few bites of goblin as Bilbo was slowly making his way over, so he wasn't hungry enough to be agressive.
Why is Galadriel more ready to believe Gandalf about the threat of the Necromancer and Sauron's return? Because she knows better than anyone that he's still out there, due to resisting his mental warfare for centuries. She knows his presence is still quite real in Middle-Earth.
Of course, Galadriel has been in Middle-Earth longer than anyone else on the White Council (except Círdan, who is considerably older still) - she knows the signs, and may even have foreseen via her mirror that this chain of events could lead to Sauron's return.
There is probably also an element of the fact that she knows Olórin and Curunír far better than pretty much anybody else left in Middle-Earth, having potentially known them back in Valinor (and most of the other remaining Noldor to have had the opportunity to do so having either left already, died, or been born in Middle-Earth like Elrond was), so she knows that Olórin/Gandalf is probably wiser and thus more trustworthy of testimony than Curunír/Saruman (the former having apprenticed under the Vala of Mercy and Wisdom, the latter under the Vala of the Forge, to say nothing of Olórin's overly modest and humble tendencies and Curunír's overlarge ego meaning Olórin is more likely to fail to bring up something relevant than to bring up something incorrect and Curunír is likely to dismiss something relevant if he isn't involved).
Adding to the above, in the book version of Fellowship there's a brief line of Galadriel's which states that she was the one who organized the White Council and had wanted Gandalf to be in charge rather than Saruman (presumably Gandalf declined and/or Saruman charmed his way into the position). That should tell you right there which wizard's judgment she trusts the most.
Much is made of Radagast's badassery in fighting the Witch-King at Dol Guldur. The reason he defeats the Nazgûl lord may well be because Sauron is still extremely weak and his servants have not regained nearly enough of their full strength.
There are mentions of how there are a number of glimpses of Thorin's deep affection for his nephews, Fíli and Kíli, in the films. Now, if you read Tolkien's anthropology notes on dwarves, it follows: because male dwarves outnumber females 3 to 1 (and many choose never to marry, just to make it worse), then even before the fall of their Kingdom, the vast majority of male dwarves never married or had children. For this reason, it follows that uncles would have great emotional attachment to their nephews and nieces.
Bifur's axe-in-the-head at first appears as nothing more than a quirky way of distinguishing the character. But in the novel, Bifur barely has any lines at all. So Bifur's main character trait is more than just that - it's a clever and probably unintentional bit of brilliance on Peter Jackson's part since Bifur is just as mute in the film as he was in the book.
On first viewing, the ramshackle mechanisms, suspension bridges, and platforms of the goblin settlement are just a cool place for a frantic running battle. Then, you think back to how the earlier views of Erebor at its height looked — awesome stone bridges spanning deep chasms, great machines for smelting and forging, rope-suspended miners chipping at gold seams — and you realize it's another case of Tolkien's worst villains being incapable of true creation, only of making shoddy copies of their betters' handiwork.
Interestingly, the novel mentions that goblins can tunnel as build as well as all but the most skilled dwarves, but only when they have a mind to, and most of the time they just use slave-labor or other means of saving time and effort. That puts a very clear light on the distinctions between the ramshackle "good-enough" quality of Goblin-Town and the loving artistry and architecture of Erebor, not as a question of skill but of attitude.
Both the construction of Goblin-town (why the town is constructed the way it and how it holds together/falls apart just as needed by the plot) and the lengthy fight and chase scene between the dwarves and goblins seem rather...improbable. The author (Bilbo) who is purportedly retelling the adventure is also notably absent, since he's dealing with Gollum down below. So who gave Bilbo the blow-by-blow account of the battle for him to describe later? Considering the colorful descriptions of dragons and orc raids so enthusiastically provided by Bofur, Fíli, and Kíli, as well as Gandalf's comment about good stories needing "embellishment," is it any wonder that the account of Goblin-town is a bit larger than life?
Bifur, Bofur, and Bombur are the only dwarves not descended from a noble line and in contrast their clothing is much browner and more bland than the rest of the group who wear colorful fabrics and furs.
While the Elvish and Black Speech in the films is all subtitled, none of anything the dwarves say in Khuzdul (even Bifur who speaks only Khuzdul) is translated for the viewers' benefit. This makes sense when remembering that Aulë made the language unique to his creations and thus they are jealously protective of it, rarely speaking it to anyone but fellow dwarves so that there are few in all Middle-Earth who can read/understand it. Tolkien didn't even expand on the language that much for the purpose of leaving it secret.
Each race has a very noticeable style for their weapons and armor - angular and broad for dwarvish weaponry; humble simplicity without a broad hilt for the men of Rohan; and notably, curved blades, usually backswords, for the elves. And, just as notably, most orcish swords are curved backswords. It might seem like the orcs are just copying the elves... but then again, The Silmarillion points out that it was Morgoth, creator of the orcs, who first instructed the elves in the making of weapons of war... Plus, the orcs themselves are elves, tortured and corrupted by evil. The similarity in their weapons serves as a small hint to that.
Why are the dwarves so mean as to eat everything in Bilbo's larder without asking first or respect to Bilbo? Well, if he's probably coming with them then they don't want all that good food to spoil, do they?
They were invited for a dinner, weren't they? Gandalf probably used food as a deal-breaker, considering that dwarves can't have been terribly enthusiastic about a detour for the sake of a hobbit burglar. Clearly a wizard's word has more weight than the guy who actually lives there.
There is the tradition that the host provides for his guests in the event of a party, however unexpected and that they be as respectful as guests; hence, the cleanup at the end where one could hardly tell they'd even been there if not for the pillaged larder. There is also no indication that they wouldn't have reimbursed Bilbo for the loss, especially when one considers how enormous one fourteenth share of the treasure of Erebor is. The fact that Bilbo doesn't take his share is mitigated by the treasure he does gain from the troll hoard on the return journey which would be more than enough to keep the whole of Hobbiton well fed for a very long time
There's also a slightly more prosaic reason in the fact that Bilbo was on his adventure for a little over a year, at least according to the book's version of events. Not everything in his larder would have lasted that long without spoiling or getting eaten by mice, so it may have been a subtle way for Tolkein to imply Bilbo was going to be away from home for quite awhile.
The Goblin King's crown had several long spikes that curve inward. It's pretty clearly a crude imitation of the crowns worn by the Nazgûl, which in turn resemble the spikes on the helmet of Sauron, himself. It's interesting to see how cultural icons are shown to pass on even among the evil races of Middle-Earth.
There was a lot of discussion about Kíli having only Perma Stubble and not a full beard when the promotional materials first came out, but considering he's an archer, it makes sense. Archers typically do what's called "kissing the arrow", which means bringing it near their mouth as a guide, and having any kind of beard would mean that it would probably get caught in the bowstring whenever he'd let off a shot, so it makes sense that, dwarf or not, he'd keep a very short beard to prevent any accidents.
Azog's oath to break the line of Durin gives a much stronger reason for the Battle of the Five Armies, while the book only gave the reason of avenging the Great Goblin's death. And he will partially succeed at it.
Dwalin is the first to arrive at Bag End. Bilbo, being both polite and timid, lets him in. Then comes Balin, who more or less lets himself in once he sees his brother. Then it's a set of young and excited brothers who overwhelm Bilbo. Then the rest all arrive with Gandalf, at which point Bilbo is hopelessly outnumbered. It's a funny sequence of events, but also almost the exact same method Gandalf will use when introducing the dwarves to Beorn.
The reactions of the dwarves to Bilbo tells you a lot about their character. Dwalin is contemptuous of Bilbo and brusquely orders him around. Balin is a bit more polite and treats Bilbo more as an equal rather than a pest. Fíli and Kíli are cheerful and seem more curious about Bilbo, probably having never met a hobbit before.
Elrond spends the movie in a much better mood compared to LOTR. Not only is it because the world's fate isn't at stake: his daughter is spending time in Lothlórien and Aragorn is only ten - he doesn't have to deal with those complicated family issues yet!
On the Fridge page for LOTR, people noted the Uncanny Valley quality of the Ring never bouncing when it falls. Now watch The Hobbit and see that it bounces like a normal object. It doesn't yet have the weight it will have in the sequels, both metaphorically and literally. Also, maybe it wanted to bounce. The rings abandons Gollum, after all. It wanted to get away, so it bounced.
Thorin, Fíli, and Kíli look less dwarvish than the rest of the dwarves, being closer to a human appearence. Why, you ask? Because the film makers want the audience to like them as much as they like Bilbo and Gandalf, aspiring to make their deaths at the Battle of the Five Armies more dramatic. It's easier to relate to human-looking characters.
The scene in which Thorin takes up the oak branch and cuts off Azog's arm is visually quite similar to the scene when Isildur takes up his father's sword and cuts off Sauron's finger. This may turn out to be significant later on.
During the scene where the Eagles rescue the Company, his gauntlet with it falls off when they pick him up. That's how Azog is going to be able to keep chasing after him as his Wargs will now have Thorin's scent.
The underground orcs are poorly armored and thus easily killed. Therefore orcs could never be a total threat on their own, without Sauron / Saruman. This is what Gimli later means under "Rabble of mindless orcs". By contrast, Uruk-hai (From The Two Towers) are better armed and armored - "Their armor is thick and their shields broad" - and therefore are much more formidable opponents.
The scene when Bilbo spares Gollum becomes this when you realize that it is a call back to two scenes earlier in the film. The first is obvious when Gandalf tells Bilbo that true courage is knowing when not to take a life. That one is obvious. the true brilliance is that it is also a call back to the scene between Gandalf and Galadriel when he says "Saruman believes it is only a great power that can hold evil in check. But that is not what I've found. I've found it is ... the small things. Everyday deeds of ordinary folk that keeps the darkness at bay. Simple acts of kindness and love." In the end, Biblo's simple act of kindness towards Gollum is ultimately what defeats Sauron.
In relation to the above Fridge Brilliance regarding Thorin's relation to Fíli and Kíli, Bilbo's adoption of Frodo mirrors the dwarf tradition of fostering relatives as surrogate sons in lieu of biological sons since very few dwarves have children.
Tolkien, unsurprisingly, knew exactly what he was doing when he picked the sign for Gandalf to put on Bilbo's door. The sign is the Norse rune Fehu, which in Norse mysticism stands for possessions won or earned, luck, a sign of hope and plenty, and success. In short, all the things that the Dwarves hope for in their adventure. The sign reversed refers to a loss of personal property, greed, and discord—in other words, a short summary of the fall of the Dwarf kingdom. It also refers to poverty (which the Dwarfs are in) and dullness/cowardice (which Gandalf wants to jolt Bilbo out of). In Middle-Earth, it matches the letter G — G for Gandalf.
There's a few scenes that seem completely unconnected to each other and are random and left unexplained. One scene is of old Bilbo and Frodo discussing their more unsavory relatives who want Bilbo's money. They seem to think he owns a cave of gold, but instead it is merely a small box. One wonders why they would think this, but if you watch the movie carefully, it's clear about how they got this impression. Bilbo hasn't told the full story to anyone and is now just writing the tale 60 years after the event. Everyone heard snippets of the story and made assumptions. For instance, in Bilbo's contract, he was promised 1/14th the amount of profit from the adventure if there was any. The Kingdom of Erebor is the richest kingdom of all time and his adventure was successful. In addition, his story included the trolls that were turned to stone. In that cave was buried a small box of gold found with the trolls that the dwarves were saving a hidden stash. "An investment for the future." His relatives heard snippets of the tale and assumed that Bilbo hid his treasure from Erebor in that very cave and is hoarding his millions of pounds of gold out of selfishness when, in fact, all he got was the small chest. It's really a nice Continuity Nod in the making with a dash of Foreshadowing. Bilbo declines his reward.
In the second battle between Azog and Thorin, Thorin gets hit by a lot of attacks that he should have been able to dodge. When you think about it, Thorin didn't get any sleep the previous night, spent all day running from and fighting goblins, and spent part of the night running from wargs and orcs. He was too tired to be agile, and thus was curbstomped.
When the other dwarves arrive on the scene, Smaug goes from talking to hunting, as noted on the main page under Idiot Ball. However, clearly Smaug knows that talking is a singular weapon; attempting to use it in the company of many would not be wise, as merely one unaffected person could destroy his manipulations with the right words. Plus, with few exceptions, Smaug's patience/restraint suffer from Gold Fever and a Berserk Button - so he sees the entire Company as a threat to that, and the time for talk is over (for now). A lot of dragons are also portrayed temperament-wise as cats, arrogant, graceful, and mean. Smaug was behaving like a cat and "playing" with his prey. Since he had already destroyed a dwarf kingdom in his prime, he clearly felt a handful of dwarves would be easy to take care of and wanted to have some fun before killing them.
Smaug quickly realises that Bilbo is invisible due to the power of a Great Ring and easily tracks him down by smell and sound. Since four of the Seven Rings were destroyed by dragons, it's possible that he has first-hand experience of their power and knows how to deal with it. Whether or not he's figured out that Bilbo's Ring is the One is another matter, though.
Bard takes twice the usual payment to smuggle the dwarves into Laketown... but that's not because he's greedy or it's the usual "smuggler's fee." It's because he needs the money to buy the fish used in the disguise, and bribe all relevant authorities. He may even have ended up doing the whole thing for free, because of the lengths he had to go to.
Thranduil beheads an orc while apparently looking the other way. Given his injuries, which include a milky white eye that he hides with glamour, it's likely that he's blind in one eye, and has learned a fighting style that takes his limited vision into account.
The "last light on the Durin's Day" has been changed from sunlight in the book to moonlight in the movie. But the Moon doesn't have any light on its own, so the light of the Moon is indeed the last light of the Sun reflecting from it.
Why do the characters keep the same length and, for many of the dwarves, high degree of organization of facial hair throughout the films, even Kíli's unchanging Perma Stubble? It's because the story is based on Bilbo's memories, jotted down more than half a century after the fact. We're mainly seeing the characters as Bilbo remembers them, rather than tracking their facial hair growth in real time.
Elephants, which are migratory animals, form protective circles around their young when faced with danger, much like the dwarves did in Rivendell. The fact that the dwarves instinctively use this tactic really drives home the point of Bilbo's speech about how the dwarves are used to not belonging anywhere.
Gandalf's visible unease and shudder in The Fellowship of the Ring when Saruman evokes the eye of Sauron is now easy to understand: he had a very close encounter with said eye and its owner in Dol Guldur.
Why did Gandalf only shield himself against Sauron at Dol Guldur, rather than trying to end it there while Sauron was still relatively weak and at point-blank range? Part of the instructions the wizards were given before leaving Valinor was not to try and match Sauron's power for power, to avoid another Beleriand incident - he wasn't allowed to engage in direct combat, and so was trying his best to survive without attacking.
The scene with the captured orc makes it quite clear that Thorin has more than just personal reasons for dealing with Thranduil, who has broken his word in the past and shows a nasty tendency towards Exact Words in his dealings with others.
In the book, Bilbo has two well-defined sides to his personality: the Tookish side, eager to see the larger world and have adventures and the Baggins side, wanting nothing more than to be back in the comfort of Bag End. Now, what other Hobbit Ringbearer suffered from a personality split? Had things turn out differently, Bilbo could have suffered from the same schizophrenia as Sméagol/Gollum!
Another from the book: during the meeting in Bag End, Gandalf tells Thorin that he found Thráin in the dungeons of Dol Guldur, mad from the torture he was submitted to. After that, Thráin is presumed dead. But, what happened to him, exactly? Did he die while Gandalf was with him, did Gandalf leave him for dead (unlikely), or was the wizard forced to mercy-kill Thráin because he couldn't get him out of Dol Guldur?
Near the end of the Battle of the Five Armies, Bilbo is knocked unconscious while wearing his Ring and therefore is not found until he wakes up after the battle and takes off the Ring. Imagine if he'd been mortally wounded or slipped into a coma while unconscious.
Bit of a cross with Fridge Brilliance: In Desolation of Smaug, Bilbo charged at and viciously killed some kind of arthropod because it touched the One Ring with its leg. He fairly growled, "Mine," in a deeper, strange voice (with a Slasher Smile, to boot). But hold on a second. Who corrupted the Rings into giving its owner infinite greed? Sauron. What creature is also characterized by Greed and has a permanent Slasher Smile? Smaug. Both are voiced by Benedict Cumberbatch, with very dark, deep and monstrous tones (also on this side of psychotic). So it would be natural that a Bilbo under the thralls of the One Ring would sound like Sauron and/or Smaug (also coupled with Evil Sounds Deep) rather than his higher vocal inflections.
Bofur getting too drunk to make the boat out of Laketown is funny at the time, but think about the implications. If he'd left with the others, Fíli would be the only candidate to go out and find kingsfoil, leaving only two dwarves at the house when the orcs attacked, one past his prime and one incapacitated. Bard's kids probably owe their lives to that dwarf drinking himself silly.