If the first movie had been much shorter - I don't want to imagine all the whining about missing scenes (including from myself).
Actually, the scene I enjoyed most in the movie - Riddles - would almost certainly have been one of the first to cut short.
Also, Smaug should not be the only antagonist since the Hobbit book has two climaxes: Smaug's death and the Battle of the Five Armies. A final battle in which the enemy appears out of nowhere and completely unexpected is hardly a good movie climax. This is both true for the whole story (thus the need to introduce orcs early on, and let them know of the quest) and for the first movie (which is why while I didn't enjoy the warg hunt near Rivendell, I do see why it would be necessary to introduce orcs+wargs early on, and emphasize their danger beyond the next five minutes).
Also, the extended backstory of Thorin and Azog gives the conflict more background, beyond the dwarves' (and orcs'😉 simple greed for gold.