A much more expansive first film
An Unexpected Journey was obviously to have originally covered much more of the plot, ending with the barrel escape from Mirkwood and Gandalf’s confrontation with the Necromancer. Conceptually, the writers seem to have been aiming for Thematic “rhyming”, with Bilbo and the dwarves escaping from the elves just as Gandalf is captured by his nemesis, with Azog and Bolg each involved in the dual climax.
Thrain’s presence, (which supposedly will be reinserted into the Desolation of Smaug Extended Edition), and Lego sets hinting at the presence of Radagast and Beorn in Dol Guldur, may have meant a very different sequence of events on the Hill of Sorcery than what we saw in the theatre. And Peter Jackson says that An Unexpected Journey would have ended with Thorin’s company encountering Bard. One speculates, as in the theatrical version, the Lonely Mountain would have remained part of the final moments of the film.
It seems that initially, Bilbo was a more doubtful character on Bard’s boat into Lake-town (as depicted in the March 2013 preview), perhaps not really coming into his own until the company reached the Mountain; however, this character trait would now probably be inconsistent with Bilbo’s more obvious growing heroism spawned from his encounter with Azog shot in pick-ups for Film 1. Bilbo bemoaning the fact that he’s “ventured east” doesn’t seem to conform with his constantly (and willingly) aiding the company throughout the second film.
Events around Lake-town and Erebor
Originally the story in Lake-town would have been shorter: it seems that Alfrid noticed the dwarves sneaking into Lake-town and reported this to the Master, with Braga heading out to arrest the company (as was shown in DoS making-of videos). According to Luke Evans, the scene with Bard recalling Thorin’s name after finding an ancient tapestry was part of pick-ups, while the failed burglary of the armory and Bard’s unsuccessful attempt to reach the wind-lance may also have been added to the original Lake-town story (Evans stated that his character was expanded considerably in pick-ups).
The fight between Legolas/Tauriel and the orcs in Lake-town also seems to have been extended, as we can infer from Peter Jackson’s comments concerning Orlando Bloom battling a “serious orc” that the Leggy/Bolg scene was the result of the Summer 2013 pick-ups (and of course, Bolg was originally in Dol Guldur at this time and not pursuing the dwarves as in the final DoS). Initially, the scene would have ended with Legolas slaying a more-prominent Fimbul, and the orcs being driven from Bard’s house.
Finally, it seems likely that the dwarves attempt to slay Smaug would have been shorter in the duology than it ultimately became in the trilogy. Initially this scene would have fallen toward the middle of the second of two films, rather than being the climax of film 2 in a three-installment version.