Lots of writers and readers think that, in the fantasy genre, anything goes. "Outside the box," is a phrase that gets tossed around quite a lot, but it only skates the very edge of truth for quality fantasy. There are still rules when creating fantasy worlds. Things still have to make sense.
Creating Fantasy Worlds – Funky Physics
Humans read fantasy fiction, and humans are used to gravity, and the basic laws of motion: momentum, friction, and all that. Messing with physics in a fantasy world might seem like an excellent, creative idea, but it might mess with the reader’s mind just a bit too much.
If you are going to change physics of the world you are writing in, it still has to make sense. WHY is there fluctuating gravitational pulls? Why has friction been negated and things pushed just roll on forever? Change too many of the familiar laws humans are used to, and they won’t be able to connect to your world anymore.
Creating Fantasy Worlds – False Normality
There are a lot of normal, everyday things in fantasy fiction. Pseudo-medieval fantasy is rife with them: horses, wagons, huts, crops, weapons and armor. Even non-medieval fantasy uses common earthly things. And this is good. 
One problem that arises when the untutored start creating fantasy worlds of their own is that they take everyday things and call them something else. "If it walks like a duck and quacks like a duck, chances are, its a duck."
Creating Fantasy Worlds – Anachronisms and OOPA
Unless you are writing alternative reality or fantastic historical fiction, cut out the anachronisms and OOPA (out of place artifacts) when creating fantasy worlds. Be creative. Stop stealing from reality.
