I like the definition of science fiction as “possible worlds” and fantasy as “impossible worlds.” I realize there are problems with both definitions, but at least it is a starting place.
There is a tendency for fantasy to ignore logic and plausibility. I don’t mean the wild coincidences, such as the hero finding exactly the right companions for a quest or the prophecy that is only fulfilled because people try to circumvent it. I am talking about the worlds that don’t make sense. Our heroes are constantly fighting predators, but there is not an abundance of prey, is one example. Numerous intelligent species live on the same planet, and they are not widely separated yet no one species was wiped out. There are armies with no visible means of support. Even elves must eat.
Fantasy is better when it is logical and consistent, but so is all literature. There is a tendency for fantasy to ignore logic and assume there are magical explanations for everything. Yet we are supposed to identify with the basic humanity of the characters, even if they are elves, werewolves, or Vulcans.