Betcha didn't see that coming. This is going to get kind of long.
I can cite 3 major influences in the way I conceive a campaign world:
LOTR: One of the things that put the Lord of the Rings over the top for me was the extensive notes in the back dealing with history, culture, language, and family lines. That really made something special that existed beyond the story.
Conan: Robert E. Howard's notes on the migrations and history of his world (plus his notes on it's eventual fall) were also something that I have tried to emulate many many times over the years in my own campaign notes.
The Penguin Atlas of Ancient History: This is another major inspiration in that it shows maps of the Mediterranean and Europe as cultural groups move in, set up nations and empires, split up or are conquered, and disappear. I love this to the point that I have drawn up a map, photocopied it, and then drawn in my racial movements and the rise and fall of kingdoms over times. If I can find these I will post some of them.
These inspired me back when I was putting together the Iron Marches campaign years ago. It's my Fantasy Hero campaign world written up in the 90's. The freedom from the pre-set AD&D classes and levels after years of playing them was very refreshing and I thought it was a pretty thorough effort as far as maps, races, gods, and multiple magical traditions. It's all written up on paper of course (It predates widespread internet access), so it's not easily posted but one of these days I will put it all into electronic form.
So after that I had one extensively noted campaign world, plus a few other starter worlds with maps and a few pages of notes that I wrote specifically for AD&D, GURPS, and even Palladium Fantasy back in the 90's. Then 3rd edition came along after after spending most of 2nd edition playing in the Realms and in other systems I wanted to get back to Greyhawk for 3rd, and development on my homebrew worlds trickled to nothing.
Later, 4th edition came out and I got married, acquiring a few new potential players and a shiny new system all in the space of a few months. I thought that starting the kids off with the new system would let them avoid all the legacy clutter of the older versions that influences my thinking, plus it would give me an untainted view on the new game. We tried it and I didn't like the way things went, but I did put a lot of thought into a new campaign world - mostly because 4th edition has so many more classes and prestige classes, since the races are so different, and magic is so different, that Greyhawk as presented in the folio or either box set is clearly not suitable, and neither are most previously published campaign worlds. Then I had an idea...
We know much of Greyhawk's history, but we also know it's not always accurate. What if Oerth had "an age undreamed of" somewhere back before the modern history of the Flanaess was written? An age where the races we all know so well lived and died alongside other, largely forgotten races? An age when magic was more plentiful and worked in strange ways lost and forgotten centuries ago? When even the gods were different with some yet to ascend and others doomed to be destroyed in the intervening tide of years? Out of this line of thinking the Lost Empires campaign was born. More on that tomorrow.