Hmm...
Not sure if I can answer this one...
History. By which I looked at History and thought -what kinds of societies would actually allow adventurers and fantasy?-
I came away realizing that medival not only doesn't work, but isn't even actually the era DnD uses as it's base. What I found was a base of ancient world (Babylon et all) to Roman Empire. But with medieval technology...
Novels...
Barbara Hambley's Dragonsbane was the first novel I read that made me say "This is fantasy..." It was also the first fantasy novel I read as a teen. There's this seperation in my life from childhood where I read the Narnia series, Hobbit, LotR, and some DnD pick your path adventure books, along with all the usual stuff for that age, then several years where I didn't read until I saw Dragonsbane in a bookstore at age 16 I think...
I think the break happened when I tried to read Silmarillion, and found it so disjointed I put it down and went on to other things in life.
So anyway...
From Dragonsbane I went on to read a list of women fantasy writers. I kept finding the male writers not to my tastes. Too shallow on the characterization, especially of women. Most of the female writers had their own characterization problems, but it took me a few years to see it (I've away with the conclusion that it isn't that men can't women or women can't write men -but that 80% of writers can't really write anybody but a certain stereotype of their own design. The 20% who can are a mix of men and women.)
What factors shape my fantasy world design now?
I do have a list of certain things that I suspect I picked up from many of those women writers. Such as a preference for strong egalitarianism on the gender angle with many lead female roles in my major NPCs. Also a strong element of the occult in magic.
Male writers tend to write magic as super powers, or something that certain people can come to understand and master (the science is magic angle). Female writers tend to write it as mysteries or spiritual forces of nature that no one can ever logically understand.
Note the word tends... That's a generalization I've found in the books I've read, and it's caused me to go for the mysteries angle. Which means I have a -LOT- of problems using the DnD magic system and almost always find my fantasy world unable to work in DnD...
There is also a bit of the Conan inspiration in what I prefer - that primal aspect, with a good dose of beefcake and cheesecake to give it a certain feel. I also tend to prefer this sort of fantasy over the heavy armored knight variety. Armor just isn't visually fun. So I build hot tropical worlds, or heavily naval world, or a mix.
Also in the line of many of those writers I've read, I spend a lot of focus on society and relationships between people and groups. More so than setting it up for action and treasure hunting, I set it up for 'human dynamics'.
And, hey, Nightfall? I've fallen into the trap that is SL products, and found I enjoy the pit. Now I can't get out. 
