Am I supposed to feel suspense right now? I thought it was established that they can't be hurt by falling great distances?
You seriously didn't see a fucking problem with this, dude?
The goblins arrived all at once, screaming and yelling; they paused when they realized that there was no great battle, but the dwarves were all stuck in the trees. The goblins, not fearing the fire, laughed and sang a song as they piled more wood on the fire nearest the trees where the dwarves were. Gandalf climbed to the top of the tree; lightning flashed from his wand, as he prepared to suicide bomb the goblins to save the rest of the Company.
What difference does it make? The entire climax is changed. Instead of the goblins attempting to avenge the death of their King, Azog's orc pack makes a reappearance. The wargs are once again treated as little more than mounts, and not an intelligent species unto themselves. The scene is somewhat less exciting in the book; there's less combat, no leaping from tree to tree, etc. The book has this scene take place in a glade, while the movie takes place over the edge of a cliff.
My Opinion: As far as "necessary changes" are concerned, I think this scene is mostly fine. I understand the desire to have a strong, central antagonist to the first film, and having this be the scene of the climatic final battle makes good sense. Thus, if you absolutely must have Azog, this is a good scene for him to return and do battle with Thorin's Company. That means all the little changes - the wargs being in league with his orcs, the extra bits of action, etc. - all work rather well. I was pretty happy that there wasn't any singing here, too. It'd be hard to take this shit seriously if the bad guys were singing a jaunty tune.
Why were the wargs treated like dogs, though? They're as intelligent as dwarves or elves. The book has them using tactics and speaking to each other. They're a species. But in the movie, they're nothing more than large, trained, attack wolves. I guess there's nothing stopping Peter Jackson from establishing their intelligence later on, but he seems to be ignoring a detail that made Tolkein's world just a little richer.
That shit with Gandalf and the moth needs some discussing, too, but I'll save that for a later post.