Two things a writer of epic Fantasy must have at his command, precious: a vision of Faerie, and the right language -- the language of Faerie -- to talk about it.
By "vision of Faerie" I mean a fairly clear idea of the fantasy world in which the action takes place. It need not be wholly of the fantasist's creation; indeed, it should not, but should rather be a melding of elements that are already familiar to the reader. It is in the melding that the fantasist should offer something unique. Readers of LOTR may well recognize the Norse names, the Finnish elements of Quenya, Christian themes and symbolism, the quaint Englishness of the Shire. In my case at the first reading, I did not really recognize those things, but their sound and use touched something strangely familiar in my cultural subconscious. As an inheritor of the English literary tradition, I had a connection with the story without fully realizing it, and it took years to figure out why. But it is Tolkien's own vision of all those things, his own "soup" of Faerie, if you will, that really puts his solid, original stamp on otherwise gossamer Faerie itself, and not just pass through as just another imitator.
The "language of Faerie" is difficult to nail down, and even more difficult to use without sounding pompous or even unintelligible. Readers of The Silmarillion know whereof I speak. It is the "grand old style" of epic fantasy, the language of William Morris, a sound that harkens to a time that never really was, but that we feel should have been. One cannot speak effectively of the wars of the Noldor, or of Numenor, without it; and yet it is, unfortunately, beyond the ken of the casual reader of modern literature. How, then, does one speak of Faerie, which by its very nature is ancient and archaic (to us), and yet remain understandable to a modern audience? To use it is perilous. It is ignored to the detriment of the Faerie-story being told. Either course risks alienating the reader.
But of course it must be used. To not use it would be like John Grisham not using "legal-ese" in his novels. How, then, does one learn to write in the language of Faerie, the language of epic fantasy? I am still not sure. Total immersion in the fantasies of William Morris has been my only recourse: reading with my handy-dandy notebook nearby, trying to build a vocabulary of English words and phrases that fell out of everyday use (at the latest) a hundred years ago. I may not be fluent in Faerie, not yet; I do not use it every day, and as anyone who has tried to learn another language knows, if you don't use it, you lose it.