I think there is going to be one major problem with the Hobbit, primarily for those (pretty fanatical) Tolkien purists who decried any changes whatsoever made to Tolkien's story of the Lord of the Rings.
The problem is this. If the Hobbit is made it will be made solely because the LotR made mega bucks at the box office. That being so, the movie studios backing the film will want a film that is like the LotR. And the Hobbit is not that film. The Hobbit is aimed at a much younger audience and that is an audience that would not bring in nearly as much money. The studios will want it aimed at the same audience that LotR was. That will mean darkening the story line that Tolkien wrote. Having seen the LotR Gandalf they will not want to confuse people with the Hobbit version, who is altogether far more prickley.
For my money the following may well happen;
1. Gandalf will be as per the LotR.
2. The number of Dwarves will be severely downgraded. A 2-3 hour film is not going to be able to give seperate personalities to 12 different Dwarves. I would think we will be left with Thorin (obviously), Balin, maybe Dwalin, possibly Bombur (if they are going for comedic elements), Gloin (for the "fans" ref'ing Gimli) and maybe Fili and Kili, so that they can be killed for Pathos at the end. That would leave somewhere between 4-6 Dwarves which would be a far more manageable number.
3. Talking trolls. Hmmm, this will be extremely difficult to handle after the trolls depicted in the LotR. I can see this being changed to an action sequence.
4. Misty Mountains. This can remain pretty much as is, but I think would be expanded to a large action sequence similar to the Moria sequence.
5. Wargs and the Eagles. For starters the Wargs won't talk. And unless they severely ramp up the number of Orcs and Wargs present, filmgoers will be unsure of why Gandalf and a handful of Dwarves can't kick their asses. After all they will already have seen Gimli going through Orcs to the dozen in the LotR. Additionally the introduction of the Eagles is not as vital as it could be because we have already seen, in LotR, that they help Gandalf and participate in battles. So this sequence could be cut.
6. Beorn will be cut or altered considerably. The scene as is may be seen as too comic, and if there are less Dwarves may not make as much sense. And an immense bear (as seen at the Battle of the Five Armies) may be seen as too implausible. (though given giant spiders et all who knows)
7. Mirkwood, expanded and turned into an action scene. Will they keep the Elves treating the Dwarves badly? Hard to say.
8. Laketown, briefly shown I should think.
9. Smaug and his death will be shown of course.
10. Battle of the Five Armies will be shown, maybe without Beorn and the Eagles though.
That would be my view of how it may pan out.
