They are going to add things alluded to in the Hobbit and in the appendices to LOTR, for instance the White Council, the "Necromancer" in Dol Gildur, stuff like that, that sets the stage for LOTR.
There are things mentioned in the Hobbit as asides that Tolkein later went back and explained as major plot points leading to LOTR but he never dramatized. Jackson will dramatize those.
Beyond that I have no doubt that he will indulge in some ridiculously long action sequences. Eg., the stone giants are mentioned almost in passing with the party approaches the Misty Mountains but from the trailer it looks like Jackson has made that into a major action sequence.
It could get a bit self indulgent, like his King Kong remake, which was ridiculously long with these dinosaur chases and such.