Differences between modern Fantasy and ancient myths and legends

Even the most fantastic myths, legends and fairy tales differ from modern fantasy genre in three respects:
1) Modern genre fantasy postulates a different reality, either a fantasy world separated from ours, or a hidden fantasy side of our own world. In addition, the rules, geography, history, etc. of this world tend to be defined, even if they are not described outright. Traditional fantastic tales take place in our world, often in the past or in far off, unknown places. It seldom describes the place or the time with any precision, often saying simply that it happened "long ago and far away."
2) Supernatural in fantasy is by design fictitious. In traditional tales the degree to which the author considered the supernatural to be real can span the spectrum from legends taken as reality to myths understood as describing in understandable terms more complicated reality, to late, intentionally fictitious literary works.
3) The fantastic worlds of modern fantasy are created by an author or group of authors, often using traditional elements, but usually in a novel arrangement and with an individual interpretation. Traditional tales with fantasy elements used familiar myths and folklore, and any differences from tradition were considered variations on a theme; the traditional tales were never intended to be separate from the local supernatural folklore.