They haven't explained it yet. They'll likely just say that they faked his death to rally the Avengers. Or, at least, they THOUGHT he was dead at the time, but he wasn't really. In a world with gods and aliens and monsters, I'm sure they'll find a way to bring him back.
I was about to say this looked really average however at the end I realized it was for a TV show and not a movie.
In which case it may very well be excellent. Firefly was an awesome show and Joss Whedon is obviously incredibly talented at creating compelling characters and throwing them into highly entertaining and sometimes thought provoking scenarios. His name alone means I'll check this out.
Speaking of Joss Whedon, is Buffy the Vampire Slayer any good? I've never watched it, primarily because the name of the show is Buffy the Vampire Slayer however I know it got good reviews and if I'm not mistaken the entire series is now on Netflix. Worth getting into?
@Oldmanwinter The first season is "Monster of the week" crap, but once you get through that, you have one of my favourite shows. There is great characterization, and the crossovers with Angel are wonderful.
I still maintain that Buffy had the BEST finale of any show I have watched.
The show goes through some ups and downs near the end, but hey, the musical episode is packed in there, along with the great finale.
@Oldmanwinter - I'm not a big Whedon fan personally. I really enjoyed Dr. Horrible, and I thought Firefly was a little better than okay (really, I thought Firefly was pretty great but with a lot of missed potential/character missteps, and not because of the cancellation), but Dollhouse bored me, and I absolutely hated Angel. As for Buffy, I feel like it's his most inconsistent series (though to be fair, I never finished Dollhouse or Angel). Some seasons are awesome, some are terrible. The first three seasons are all decently solid to great (though not without their poor quality moments), but Season 4 made me quit the show. I eventually forced myself to come back to it, and then Season 5's opening made me quit the show again. Once I forced myself back a second time, it was relatively smooth sailing throughout the remainder.
Since you seem to hold Whedon and his writing in a higher regard than I do, you'd probably really like Buffy. I prefer Buffy to Dollhouse and Angel (though most people seem to prefer Angel between it and Buffy), but it doesn't hold up to Dr. Horrible and Firefly for me. Still pretty good overall though.
On-topic: I'm looking forward to this. None of the new shows so far have totally blown me away (though Mind Games and Resurrection may have me slightly more interested as far as the new ABC shows go anyway), but this does look like one of the more interesting ones. I do hope they come up with a better story for Coulson's return than a simple "Loki's attack was never fatal, and we just pretended he died to motivate the Avengers" thing though.