At last, the Lost ending explained by Ben Linus

SPOILER ALERT if you're still watching the show from 2010.

Michael Emerson, the actor who played the morally ambiguous Ben Linus on mystifying SFF TV series Lost, has finally explained that the characters are not dead, as many assumed, but in a sort of ante-chamber to full-blown death, as it were. To progress to the afterlife, they need to go two-by-two into eternity.

"They’re all gonna pass through to a happy afterlife. Just as in a Shakespeare play, everybody goes two by two. It’s couples. That’s because, I think, by the rules of Lost , you can only pass into heaven - if you want to call it that - with a mirror redeemer. With someone who has loved you without reservation, for yourself. Everybody had that I think, except for Benjamin Linus. That’s why he can’t go. He has to wait; he needs to find his mirror redeemer."

So they weren't dead (except in Season 6 flash-sideways timeline).