One of my favorite books and movies is "Alice in Wonderland", or "Adventures Through the Looking Glass". C.S. Lewis was an incredible man, amazing author and devoted Christian. He wrote from a place of wonderful imagination, setting a standard for creating alternative worlds. In my humble opinion he was one of the most influencial fantasy writers in history.
Writing fantasy is a subjective area. Not everyone is going to 'get' it, some are going to be agast, a few might even get indignant - does that mean you should stop what you are creating? No.
The process of creating a fantasy world is vast. Geography, history, languages, people, even animals and trees have to be reconsidered in light of your world. The trick is to not create a world so different from present reality your reader has too hard a time following what you have written.
In the months prior to beginning "Catalyst", Book One of my Guardian Series, I carefully constructed a post-nuclear earth. The crust had shifted from the stress of nuclear warfare. The people have changed from centuries confined inside the Sanctuaries (a post for another day). Everyone has some common ancestry yet not so much as to assume only one people group survived to replinish the earth.
One person's fantasy is never anothers, so I encourage readers to branch out from their familar and favorite authors to embrace something new. Mankind has a thirst for adventure, a desire to be lifted out of their everyday lives even if just for the length of a good book.
So write what your heart tells you to write. If it means creating a new world, then make sure you enlighten the reader to the wonders and dangers of that realm as you see them. If you are using our current time and home, be sure to use places and situations you know best. If a reader thinks you don't know what you are writting about - the critics will slice you up and serve for you for afternoon tea.
And remember -
"Beware the Jabberwocky my son"