oh, I have neglected this thread...nice ones, folks.
I also loved the world of "The Sword in the Stone" - (more than the rest of the books in "The Once and Future King") -
Creation of other worlds
I fogot about Lemony Snicket! Those are great fun, although I guess they aren't exactly "other worlds". But who cares? What a fiendish imagination!
They might not be other worlds, Tomkitten, but I've never been in a Snickety place!
Creating other worlds
My feeling is that there is a difference between other worlds, and other places. Other worlds are not of this world, imaginary, unreal. Other places are simply places and/or times in which the reader is not. So I would call E. Nesbit's England, another place and time, but not another world, even though I could never be there myself.
It takes imagination and at least a certain amount of knowledge to make another place convincing, but to make a whole world convincing, when it is like nothing that ever existed, takes a whole lot of imagination, and a great deal of logic. It's not easy to dream up a new world whose features, whether geographical or cultural, hang together and form a convincing whole.
All that being said, I ask: is a parallel universe another place or another world?
J.R.R. Tolkien uses the term "sub universe" implying a different world dependent on our own.