PeterM, the stone giants are actually in the book itself, but they didn't play a big role. Of course, in the movie the scene was big and fully fleshed out but if you look at the story itself, it only represented a few minutes of the characters' experience so I don't see that as a big deal.
For adding Azog as a BigBad, I don't think it took much away from the story, since the big scene fleeing the orcs and hiding in the trees was in the book and it's really not a big change to the story to make the leader of that raiding party identifiable rather than just some random commander. The whole scene with Thorin charging off the burning tree was fluffed in, but taking that out wouldn't have cut more than ten minutes off the length of the film and it gave Mr. Jackson a chance to show Bilbo's acceptance into the dwarves' group outright rather than trying to demonstrate the more subtle way the book handled it, so again I don't see it as a particularly awful retooling of the story.
As to fleshing out the story for the money, I get the feeling that it was Peter Jackson saying, "I can make a trilogy so I can do the story the justice that I want, and I can point to the box office for the last trilogy to convince the bean counters that it'll be worth it." I have no doubt that the studio approved a trilogy because of the money draw but I see that more as giving Mr. Jackson his head because he's bankable than any studio exec starting out with that plan.