I'll probably write a hub about it, but fantasy art allows for a merging of artistic sensibilities. Some of the basic elements, off the top of my head: 1) It allows an artist to draw from the imagination as well as from observing the world around us. 2) It borrows from sequential art in the sense that it tends to emphasize movement in dramatic ways. 3) It allows for exaggeration
Although fantasy art allows for breaking some rules, it must have an integrity and "believability" of its own. A herioc figure in an exotic landscape must be grounded enough in reality for us to relate to it--otherwise it invokes dream (or drug-induced images) instead--a whole different ball game.
Well--those are my surface thoughts.