It was probably only a matter of time before someone took two of Joss Whedon's most beloved creations—his late, lamented sci-fi western and his recent, ginormous Marvel movie—and crammed them together. And damn if the result doesn't just make you want more of both.
In retrospect, the wistful Americana of Firefly's country-esque theme song goes perfectly with the story of Earth's Mightiest Heroes who don't want to be told where they cannot stand. And, yeah, the Helicarrier does seem like a ship that's kept in the sky by little more than hope.
And yet ... what the heck is Tom Hiddleston's Loki doing there? Villains don't get in the opening credits.