Read through all the REH stories at least twice, and as other members have echoed, none of the other authors, not Lin Carter, de Camp, John Maddox, etc, even neared the feel that Howard conveyed in his stories. From reading those pastiches/fan-fics, I got the sense that the authors' visions of Hyboria were not as clear-cut as Howard's, and that there was too much exposition on the narrative and from the mouths of the characters, leading to far less "treading on jewelled thrones" and more "he cometh and conquered and kicked ass". There's far less left to the imagination.
Howard's writing on the other hand has a certain naivety, with most major characters having strong convictions in either the gods or power. He does not focus on minutiae, instead devoting his attention to either describing evocative details to convey emotions to the reader or to drive the story forward. Think about how he describes Zamboula or Fort Tuscelan or the Poitainian fields. In every instance, his style surely evoked not just the appearance, but also the mood, mannerisms and way of life in those places. All of these things I thought were lost in the pastiches.