SPOILERS! SPOILERS! SPOILERS! ALL AROUND THIS POST!
It is explained by the Catalyst. Shepard dies, because his organic DNA is used to combine it with synthetic parts of the Crucible, something like that. He is annihilated.
I like how they added some parts, small choices, like you can convience Illusive Man that he is indoctrinated, they added some "explaining" dialogues for the Catalyst etc etc.
And the new rejection ending is good for people who didn't like old endings at all.
I don't understand how Shepard survives the red ending, though. After Cerberus made him from scratch, he's in many ways synthetic. He should die.
END OF SPOILERS
Good, I went straight for the ending animations. What really bothered me in the original endings was that, IIRC, Shepard was only supposed to die in the Control ending, and the relays were only supposed to be destroyed in the Destroy ending, at least according to the original dialogue with the Catalyst. It's good to know they've actually made it explain the consequences a little better.
About the new ending, though, it would be better if it was possible to actually win the war. It could be said that no other group of civilizations in the galaxy had ever been as united as the current generation, thanks to Shepard. Plus the Reaper's initial surprise attack through the citadel had been denied, the alliance had copied their Thanix cannons, etc. Costs would be heavy, but there could still be a chance.
Finally, I can say I'm satisfied.