I noticed the dark feel too. Really looking forward to it. The trailer gave me the feel that he realized there were unstoppable forces out there. Things he couldn't stop alone. Which, before Avengers, he had pretty much handled every situation alone. To an extent, at least. And for the first time he had to rely on a monster, a super-soldier, a god, a couple of assassins, and all of SHIELD. So, for once, he's actually concerned about the fact that the world knows who he is. And I really like that. It has a good tone, and so far I'm interested in and curious to really see Ben Kingsley as Mandarin. I hear the movie is also supposed to explain why War Machine wasn't around for the events of Avengers.