What I really don't get though is why they'd spoil the fact that he has a weak spot on his belly before the dwarves even get close to the mountain? For anyone not familiar to the story it makes it painfully clear waaay too early how he'll be defeated. That and changing his awesome armor-bling, although I guess they might've had issues making it look good, what do I know 
One minor thing about that weak spot that gets to me: wouldn't Smaug at least try to put something there? With that much stuff on his trove, you would think he would have found something in all the years he was there.
It's implied he didn't notice.