If you are a fantasy writer, you NEED to watch this video! That means you @Mr_NutterButter ;)
"What is fantasy without mystery?"
It is something I definitely wanted to incorporate into my world, and although I know a lot of answers to a lot of mysteries they may never actually be outright explained. In fact, I'd say a great majority of them will never directly be explained. I always try to tell me D&D players "this is what your character would know" or "this is what the different beliefs are" rather than a definitive answer. It makes it way more interesting.
Why isn't magic used more? Why is it just now becoming more popular and potent? What was the actual cause of the plague? Who is a real god and who isn't? What power do the flowers have exactly? What in the hell is the Old World? Those are just the more grand questions. There are so many more little intricacies that I love.
I like to see the players' interpretation of things that I've made, and watch them navigate their way through them. If I explained everything, that would be totally lost. It's why I love being a Dungeon Master more than a player. A writer more than a reader. A game developer more than a gamer.