"It is so real in my imagination that I could go Google Oz with it just like Google Earth. I can zone in to any little corner and find something fascinating. The place feels so real, with its own history and population, its peculiar strains of beliefs and imagination and social progress. It's the vehicle that has allowed me to open up the most far-seeing apparatus of my imagination."
I too have the ability to conceive in my imagination entirely new worlds. Since childhood I've imagined vivid, detailed worlds, thriving in my mind, rich with imaginative, created reality. My obsessive-compulsiveness also leads me to imagine all sorts of scenarios occurring in my life, all sorts of situations, all sorts of fantasies, hopeful or frightening or tedious. In my mind, all sorts of events and places and people have existed, created but not real.
And I don't assume I'm special in this way: I think many, if not most, if not all, of us are capable of creating worlds in our minds. Of imagining that which is not with rich detail.
What I lack, and what most people lack, is the aesthetic ability to express my created worlds to others. I may be able to convey what I imagine, but not with eloquence or beauty or real art. I cannot express it well in fiction or poetry, nor do I have much ability with visual art. I don't believe I have aesthetic ability (though maybe someday I will find it, I don't know). I can create worlds in my mind, but I cannot artfully give my world to you.
And this is what separates a writer's imagination from a non-writer's imagination (I imagine). It is not the ability to create, but the ability to aesthetically express that creation to others.
(And in my opinion, anyway, Gregory Maguire does have that ability to aesthetically express his created world).