There was so much potential there for years of stories, but it would have required a tremendous amount of committed effort. If Kripke hadn't insisted on sticking to his "Swan Song" guns, we could have had a truly evil Sam, a real apocalypse and some unbelievably kickassery from Dean and Castiel. They had the chance to make this show as complex and challenging as Battlestar Galactica, and I think another team of writers would have gone for it, but this one clearly didn't want to do anything that risky.