What are the big moments that thrill? (The story beats, I think they call them.)
1)Gandalf bullying Bilbo into being the Burglar.
2)Outwitting the idiot trolls.
3)Escaping the orcs in the Misty Mountains.
4)Bilbo getting the Ring
5)Escaping the spiders in the forest.
6)Escaping the elvenking's dungeon.
7)Bilbo confronting Smaug.
8)Bard slaying Smaug.
9)The Battle of the Five Armies, culminating with the advent of the Eagles.
10)The death of Thorin Oakenshield.
1&2 are awfully cutesy, unbefitting the gravity of the Gandalf and Bilbo we've seen. And 6 has a drunken elf, which sure doesn't match the elves we've known.
3, 5&8 are closely paralleled in LOTR, with the escape from Moria, the escape from Shelob and the final battle at the Black Gate, except the LOTR scenes are bigger and more consequential.
We've already seen the highlights of 4!
For 8&10, we have to care for Bard and Thorin for them to work. They've got stiff competition in Eowyn killing the Witch-King and Frodo's death scene in LOTR. But they may work.
The only purely new moment that promises to be intense is 7. That's a little short for two movies.
They've got to stuff this sucker. Whether it'll work is unknown as yet, of course. But don't kid yourself about the difficulties in making this thing watchable.
A straight up adaptation of The Hobbit as a normal length kids' feature may have be the artistically sensible choice.