IMHO, credibility in fantasy is second only to the fantasy elements in importance. I think that is one of the keys to the success of The Hobbit and LOTR brand; it could have happened.

The first thing that kills my interest in reading a fantasy book is hitting something that makes no sense. An example, in one book, gods are riding horses through the book. Why would gods need to ride horses? Shouldn't they be going via their own godly powers?

One can attribute much to 'magic' as in spells, but if the fantasy element is 'out there,' then give me a clue as to how it is possible. For example, a character 'goes to another plain.' No one has yet figured out how to do anything like that. Give the reader a hint as to how it could happen.

A third thing that bothers me in fantasy is jerking me out of the scene to tell me what is happening. Fantasy is escapism, pure and simple. When I read it, I want to go to that world, be immersed that scene. I want to be involved in the action, feelings, and thoughts. Show me what is happening, don't tell me from an outside POV.

For fantasy to impress me, I need to feel and believe it could happen.