Some time ago, Poul Anderson wrote a famous essay providing advice for would be authors of heroic fantasy. The title of the essay was "On Thud and Blunder" and that title became a descriptor for an entire sub-genre of mediocre and derivative heroic fantasy stories. In written form, "Thud and Blunder" tales would include the John Norman Gor novels (though those have additional issues as well), the Lin Carter Thongor tales, and the vast majority of Conan pastiches. In film, almost every heroic fantasy ever made -- with some recent exceptions -- falls into the "Thud and Blunder" camp. Kull, the Conan movies, Krull, The Sword and the Sorcerer and countless other films fall into this category. The recent Lord of the Rings and Harry Potter movies (among others) have managed to avoid the syndrome, as has the wonderful independent film The Midnight Chronicles by Fantasy Flight Games. One imagines that the upcoming Conan film will be no different from its predecessors in this way. It seems that whenever anyone writes a Conan story (no matter the medium), they use the old Frazetta covers as inspiration rather than Howard's work.
As an aside, Anderson mentions DeCamp as a fantasy author who managed to avoid writing tales of "Thud and Blunder." Those who are only familiar with DeCamp's Conan pastiches might find such an assertion baffling, as DeCamp's tales of Howard's barbarian are particularly bad, but those readers would be well served to read further into the library of DeCamp's work. Sprague was quite a wordsmith and when he wasn't busy unfairly damaging the writing reputations of talented pulp era writers, he was writing wonderfully fun and imaginative fiction. One might attribute the degrading of past authors by a talented author of one generation as a necessary "canonicide" by which one generation of writers asserts its talent and authority, were it not from the genuine pleasure that DeCamp seems to derive from reading the fiction of Howard and Lovecraft.
Adding to bewilderment in this regard is DeCamp's contemporary Lin Carter. Carter also enjoyed and promoted the virtues of heroic fantasy, and compiled wonderful collections of older fantasy writings. Carter's own attempts, like the aforementioned Thongor series, are nigh unbearable to read. Yet Carter's passionate, and articulate, introductions to his collections demonstrate that he could be a capable writer.
Maybe there is something about the heroic fantasy pastiche that brings out the worst writer in all of us, kind of like buddy heist movies can bring out the worst in screenwriters.
Back to "On Thud and Blunder" though...
The key tenant of advice that Anderson, who was a skilled author of heroic fantasy, gives to prospective authors is the need for verisimilitude in the presentation. Certainly fantasy tales will violate many of the laws of nature, but they should seem to take place in living and breathing worlds. Anderson provides several ideas for areas where authors might look to increase the realism of their world and the quality of their fiction. He recommends that authors think about the physical aspects of the environment (what lighting would really be like for example), the real politics, the role of religion, the realistic use of weaponry, and/or the lives of the common classes when they approach a fantasy tale.
When one thinks about it, the best fantasy stories are those that do just that. What draws me to George R.R. Martin's epic fantasy? His portrayal of political relationships. What draws me to Michael Moorcock's Elric saga? The living nature of the metaphysics and religion of the tales. Elric's actions have consequences and the religion of his people is a "living" thing -- quite literally. Tolkien was a wonderful practitioner of mythopoesis. Even when Tolkien's tales lacked "action," they contained deep realism.
Anderson's brief essay should be required reading for any fantasy author, and for most Dungeon Masters as well. Think about how much better your role playing game sessions would be if they took place in a living world. I often think that James Maleziewski's rejection of the "narrative" module model of rpgs, is that he wants to have room for a deep verisimilitude that is often included in "geographically" based adventures and lacking in "narrative" ones.
My only criticism of the Anderson piece are his uses of Society for Creative Anachronism activities as proxy for any kind of historical representation. These events have themselves become as divergent from the reality they seek to recreate as anything else. When one, as Anderson does, begins discussing chainmail constructed of hanger wire as analogous to real chainmail it is easy to see how the comparisons can begin to fail. Add to that modern metallurgy, which creates lighter and stronger metals, and the errors only begin to compound. SCA comparisons aren't useless, but they shouldn't be viewed as "accurate simulations" any more than an episode of "Deadliest Warrior" or a wikipedia article. Members of the SCA aren't typically Andre Marek who attempt to live their entire life as if they were in the middle ages. Speaking of Andre Marek, the Timeline film is a perfect example of how you can take a book which isn't "Thud and Blunder" and transform it into a "Thud and Blunder" tale in another medium.
I'd like to re-assert though that if you want to write fantasy, or if you are looking for game master advice, Anderson's "On Thud and Blunder" is must reading.