Board Thread:Questions and Answers/@comment-30150700-20161005162327/@comment-30150700-20161006210650

Ao Kusanagi wrote: The explanation regarding this is in the very first episode/novel volume. The NerveGear helmet emits intense microwaves that will fry the brain of any player whose HP reaches Zero. There's also no stated guarantee that the revival item will work, but it is speculated that the person could be saved within the ten-second interval before the microwaves activate.

IOW, yes, players really do die in the game. Kirito and Asuna were the sole exceptions because Kayaba wanted to talk to them mostly out of respect for bypassing the laws of his world(Asuna for breaking out of the paralysis through sheer willpower, which he stated to be impossible, and Kirito for managing hold his avatar after losing all of his HP, despite that also being an impossibility, long enough to stab him). But then how could Kayaba have set it so that Kirito and Asuna didn't die? He had no way of knowing that they would perform the amazing acts that they did, so it seems impossible that he would have set it up before the duel, and he for sure didn't have enough time to enact these changes during the duel, and afterwards there would have been no time before the 10 second window closed and Kirito and Asuna's NerveGear fried their brains, so I don't see how Kayaba could have rigged it. Perhaps my favorite theory (partly due to its use of a supremely underused character imo) is that Klein uses two revival items on Kirito and Asuna. Since 2 Christmas Eve's pass between the beginning and the end of SAO, Klein kept the revival item Kirito got and got the second himself. However, it fails to account for Kirito, Asuna, and Kayba's conversation as Aincrad is breaking down, so I don't know how canon the theory turns out to be.