Those who have read THE HOBBIT understand the weight he bares on its story. He is not just a dragon. Not just a villain. His presence looms over the entire narrative, both through his monolithic physical form, and also for what he IS.
Says GDT on the TORN message boards, "Smaug should be 'The DRAGON' for all movies past and present. The shadow he [casts] and the greed he comes to embody- the 'need to own' casts its long shadow and creates a thematic / dramatic continuity of sorts that articulates the story throughout."
Some LOTR folks may cringe at what I'm about to say, but I think it is a valid comparison: Smaug is Darth Vader with scales.
That's not to say that Smaug is Bilbo's father (although weirder things have happened - did you see BEOWULF?). When I make this comparison, I'm talking really about two things:
1. When most people think of the ultimate science fiction villian - they think of Darth Vader. Some even think of him as the greatest villain in movie history (I do!). His status as a symbol of evil, a symbol of dark power, is unquestioned. When you see the old Star Wars movie posters, it is the Darth Vader helmet that is looming menacingly over the heroes. Smaug needs to have that kind of presence, that kind of stature, in the minds of movie-goers. Given GDT's quote above, I think he has the right idea. Its not just a matter of making a "cool-looking" dragon. This is about building an iconic character that stands out from all other dragons in movie history.
2. The Star Wars saga is not about Luke Skywalker. It is about Darth Vader and the ultimate redemption of Anakin Skywalker. Each character in the film is linked in some way by Vader's destiny. Smaug's impact on THE HOBBIT is similar, although not so profound. It is not Smaug's destiny that fuels Bilbo and his cohorts, it is his "essence". His greed. His pride. These traits are at the heart of Bilbo and Thorin's actions and drives the story. One rejects them, the other is enslaved by them.
(Relatively, Luke rejects power when tempted by Vader in their final confrontation. Vader's "essence", I would say, is power.)
Just a few random thoughts - feel free to comment below. I'd love to hear your take, or if you think this is a worthwhile comparison.