I don't think anything, yet, but I'll mull on this today and hopefully have some thoughts by the evening.

Great post.

Truthfully, I was worried about The Hobbit being artificially split into 2 films, and now they're making it a trilogy. So, basically even more made up stuff and almost definitely more offball dwarf humour - things which I thought were among the weakess elements of LOTR. Dare I say this smells like hubris. I just reallly hope it doesn't backfire and we end up getting 3 films that are almost immediately fan edited by everyone and their tennis partner into one 3 hour epic.

The nature of film series is that they're going to be at best uneven, since different people are going to involved and/or the same people are going to run out of ideas/steam (or worst, misunderstand why the series was successful in the first place). The longest running series (in the west anyway) has got to be Bond, and they're all over the place with some fantastic ones and some which are pretty meh.

redxavier, in Jackson's defense, he cites the breadth of the stories he's already telling in his statement about his decision to make it a trilogy:

Your mileage will vary considerably, of course.

Personally, I'm very interested by the fact that this decision is so last-minute, because it implies, to me, that Jackson and company were looking at the footage they had and thinking they had enough material to constitute three movies. I kind of like that idea, because it's been only recently that I've discovered what difference good editing can make in a film—contrast and compare the theatrical cut of Star Wars to the current DVD release, because that's what I did. 

But Jackson is in a unique position among the current trend of "let's split the film adaptation in half for no discernable reason!" because he has access to more story than he can possibly cram into a trilogy, even with just the appendices (whether or not he will use it, of course, remains to be seen). Ringer speculation before today's announcement was that a third film would be a "bridge" film, covering events between The Hobbit and The Lord of the Rings, and the production is pretty open about going beyond the story of the novel into the story of the universe. That sounds defensive, but I bring this up to differentiate this from the current slew of novel adaptations snapped in half: Harry Potter and the Deathly Hallows, Breaking Dawn, and Mockingjay. The big question here is not whether or not this is a huge marketing gimmick (which is a totally valid point; if you're a completionist Potter collection, you had to triple dip for that last film), but are audiences becoming more willing to watch longer and longer films?

Which dovetails with your question, Doctor Submarine. Yes, I definitely think the fact that the Extended Editions are so popular was an influence on Jackson, because he knows his audience will happily park their butts for well over two and a half hours and want more story. I've marathoned the extended trilogy before, which was both butt-numbing (fidgeting on a sofa for twelve hours) and dehydrating (we popped in the second disc of The Return of the King and just started weeping), and I was looking forward to marathoning it with The Hobbit before today's announcement. (I don't know if I can do six films in a day; surely, there must be some Star Wars brethen who have done this, it can't be terribly difficult.)

I just don't think the story of The Hobbit is epic enough for three movies. I was pleasantly surprised that they'd decided split it into two films because it does have a strong travel-y bit and then later a strong dragon-y bit.

I don't know. Maybe in the middle Bilbo will fight Angelina Jolie as Gollum's mother.

It's very easy to confuse backstory with story. The appendices of LOTR are appendices, they're not part of the story, they are there to add scope to the world and do not tie in with the narrative itself.

Personally, I don't want to see the Necromancer or Dol Guldur or anything of what Gandalf gets up to whilst Bilbo and the dwarves are on their adventure, because the function of those elements in the Hobbit was to serve to demonstrate that a wider world existed outside the immediate story. They're not fleshed out, indeed virtually nothing is known about them except the events themselves, because Tolkien intended them to be somewhat mysterious and to have readers a little enticed by the hints. Indeed, when I read the Hobbit I love the fact that Gandalf comes back to the group and reveals some other adventure has taken place.

As I've said before, for all its virtues Jackson's LOTR trilogy lacked subtlety and it looks like this is going to continue in his Hobbit since the little side mysteries are likely going to take centre stage. References to the "Battle of Dol Guldur" imply we'll see this and unless it's a flashback I see it getting a Helm's Deep treatment.
  How in middle-earth does that help tell the 'full adventures' of Bilbo Baggins?