Building A Fantasy World

First, an announcement: I want to let everyone know about an incredible workshop possibility for MG/YA fantasy novelists. Highlights is sponsoring a week-long, mentoring workshop with established fantasy authors. They are only taking 8 authors. Check out the details here:
http://www.highlightsfoundation.org/pages/current/FWsched_wholeNovelFantasy_10.html

I've applied and am waiting anxiously! You never know what God will decide.

We talked about Character Arc for our heroes and villians - some secondary characters should have complete character arcs, too, but we don't need to talk about at this point.

Let's talk about building our worlds. Fantasy writers have somewhat of an advantage over contemporary writers in that we can do anything we want with our worlds. We don't have to obey the laws of nature or have a world that is like Earth. Of course, we want to make our worlds real so the reader feels part of the story. How do fantasy writers create fantasy worlds that seem real? What do we include? What should we avoid? I look forward to good discussion!