It seems you hit something, Simon... And it seems it may hurt as you touch a way I'm used to MCing...
When I'm MCing Apocalypse World, I'm doing it to find out what happens. And finding out what happens begins while designing the whole landscape of the Apocalypse World we (the players and I) are going to share, I haven't a pre designed setting, even in head.
What's your apocalypse world's looking? Don't ask yourself for the origins but for the aftermath, think of it as a drawing. It's my first question and I ask it while the players are choosing their playbooks (not during the first session) as it could be a prerequisite for character design, indeed. I'm not asking this to know what the day to day life of the PCs looks like, but to have a global picture of the world. This picture is for me as a MC and for the players too.
As for "what's really the world psychic maelstrom?" question, the "what's the origin of the apocalypse?" is a recurrent one and many players (some of the one I practice) would be seriously shaken and maybe, even paralysed in their creativity if they didn't have this one solid picture to dream their character of and make them real (Nausicaä of the valley of the wind isn't mad max, which isn't fallout or borderlands).
Maybe I'm wrong not to play the world building but for some players it's simply too vague to keep on character building. I cannot stay here, looking at them and simply answer: "I dunno" as I've done. My answer is now: "I don't know, but we're going to design it now". Without this world building first, some players (many of them are very old role playing gamers) couldn't simply play the game as it requires a blind trust in the MC who they still see as a GM.
Without this world building first, some players need to create their own Apocalypse World. Sometimes, they share their creations with the MC (to gain social acceptance?), who's asking himself: "Sure... Great work, it looks good, but what's the part of the secret in Apocalypse Word?"
As I'm answering this post, I'm playing a new AW pbp game. No secret creation is allowed and I'm trying to use Microscope to help designing the world. Each player, each turn is claiming or refusing one aspect of the world.
As for now, we've got this: no zombies, no cannibals, small holdings (less than 200), holdings built in ruined cities, scarcity in finding energy, separated distant holdings, strange tribes living in between, no difficulty to find metal and new holdings are frequently born (like swarms).
I find it difficult The way you clarify how to build the setting in AW
The players don't get to say what's real in the world except as it relates to a question the MC has asked them, yeah?
. This will come while 1st session playing, but I don't wanna take the whole authority to choose what the world would be. When we'll get some global world agreement, then could I barf forth apocalyptica and answer questions like crazy during first session play.