I don't expect a great answer till all parts are released, but at least, as per the current knowledge, what elements (plot, characters, events, ideas, dialog, items etc..) are known to be in Peter Jackson's "Hobbit" movie(s), which are taken from Tolkien legendarium (e.g. LOTR, Silmarillion, letters etc...) but NOT in the original "Hobbit" novel?
Just saw it! Many Tales are woven together that are only barely mentioned in The Hobbit. I'll just quickly write that a lot of the expanded material comes from the writings that were still about Middle Earth and mostly the Appendices of LOTR. Some of the material may also have come from earlier drafts of both The Hobbit and LOTR within its appendices.
Radagast the Brown gets a much more extensive treatment than I would have ever expected and liberties were definitely taken with the timeline Tolkien indicates in his writing vs. what is depicted in the movie.) There were also additional bits that needed to occur in order to cohesively string together the other tales.
Of course some scenes from the book are shortened or deleted (as one expects from movies), but at the same time, other scenes are lengthened significantly. For example, the book mentions the hobbits only seeing stone giants playing a game in the distance. The stone giants have a much more prominent part to play in the movie - though they still have no lines.
In the book, there is only a tiny piece written about Gandalf's acquiring of the key and map and the book only mentions the necromancer twice as well as only mentions a council between the elves and wizards - these events are greatly expanded in the movie/movies. An extensive background piece on Erebor and its fall is placed near the beginning of the movie as is a bit between Bilbo and Frodo (on the same day of the one hundred eleventieth birthday party) that creates a sense of the entire movie being a big flashback. I believe that many of the smaller additional pieces comes from combining a short bit in the LOTR Appendices titled, "Journey to Dol Guldur" Though, again I am only working from memory here.
When the movie is depicting scenes from the book The Hobbit it stays fairly true to the book throughout even down to word-for-word dialogue (with the exception of a "hunt" and related additions I will allow you to watch to find out more about.) There are changes here and there such as where exactly Bilbo gets stuck and loses his buttons and some of the dialogue, but the story is definitely there. Thorin also comes to the party at Bilbo's late instead of arriving with the last group of dwarves as he does in the book. The story is just alternating with other stories too so you are taken back and forth between word for word accuracy and completely separate non-hobbit bits. For fans of the book it might be a bit jarring (it was for me).
I was disturbed somewhat by a mismatch with Tolkien's timeline of events and the one used to piece together the movie and still can't really decide how I feel about the additions -even a year later. What bothers me most is the idea of Azog hunting and chasing the dwarves throughout the entire story. Bilbo almost seems to lose his innocence and naivete just a little too soon for me as a result of this particular story arch. At the same time it enriches the audience understanding of Thorin and the plight of the dwarves. Of course it also adds opportunities for action sequences during the company's travels. Some of the light-heartedness of the story and its humor is missing.
The movie version delves into the rich complexities that make Middle Earth so engaging a place, and, perhaps the changes do make the story more cohesive with the LotR. The changes certainly make it more cohesive with the movie version of LotR. I would simply argue that you should definitely not go expecting to see The Hobbit, but rather go expecting to see an explanation of the finding of the ring. Including beautiful cinamotography and a chorus of engaging, relatable characters. Film making is just such a different beast than writing a story in books after all.
Elements of the character of Tauriel are from non-Hobbit canon.
We know that she's a red-haired Silvan Elf, so let's examine how a Silvan elf can have red hair.
There's one group of Elves in Tolkien that we know have red hair: some of the Noldor. In The Shibboleth of Feanor we learn that:
Furthermore, in Quendi and Eldar we learn that of the Second Kindred, who were originally in proportion 56 out of 144 of the Elves, 28 became Eldar and the other 28 were Avari. These 28 who became Eldar were the Noldor, but the other 28 remained in Middle-earth.
Now, Appendix A to the History of Galadriel and Celeborn gives one origin for the Silvan Elves, stating that they originally belonged to the Third Kindred:
However an alternate tradition, in the original drafts for the LotR Appendices, gives a different origin:
Bringing all of this together, an in-canon source for Tauriel is that she belongs to the Avari of the Second Kindred who were subsequently ruled by Sindarin Kings from Doriath.
A rather neat side-effect of this is that it offers another explanation for Thranduil's disdain for her in the movie; the Sindar of Doriath had a disliking for many of the Noldor as a result of the Kinslayings, and as (1) Tauriel is descended from the same Kindred as the Noldor (and especially is distantly related to the Feanorians), and (2) Thranduil has origins in Doriath, everything becomes clear.
Of course, this all uses sources that Peter Jackson is not allowed to use, so you'll probably never get official confirmation from him that this was his intention, but all of the evidence fits.
Radagast the Brown (in Mirkwood & west of Bruinen river when distracting the pursuing orc pack) and the White Council. The WC meeting (in Rivendell) is provided much later than in the books to help highlight the growing threat from Dol Guldur. In the extended version of the film there's some extra scenes at Hobbiton that doesn't happen in Chapter 1 of the book, but interesting.
Elrond has a short scene with Bilbo in Rivendell talking about the reputation of Hobbits as 'resilient' and 'enjoying the comforts of home' and also stresses he can stay in Rivendell "if that is your wish". Not in the book, but a nice addition for the extended movie version.
The Dwarves, led by Bifur, sing the Man in the Moon song in the dinner scene in Rivendell, which of course doesn't happen in the book. But nice to see it done.
Another is that the Witch-King of Angmar was killed in his war against Arnor, & buried in the 'High Fells of Rhudar' that Ganfalf later visits in the 2nd Hobbit film. In reality he isn't killed but rides away on his black horse after the Elves intervene in the Battle of Carn Dhum. Added in the White Council scene in Hobbit film#1, I guess for dramatic effect, etc.
Another is Gandalf using a moth to obtain help from the Great Eagles of the Misty Mountains near the end of H1, in reality they're attracted by the smoke & flames so come to investigate.
The "chance" meeting of Gandalf and Thorin in Bree is from The Lord of the Rings Appendix A. Gandalf had some concerns about how Sauron, now rising in power, might use the dragon.
Frodo isn't in the original text, though he might be in later versions, I'm not sure. He's listed on the IMDB page though.