But, for all of his failings as an author, he excelled at philosophical fantasy. He had an amazing imagination and he communicated more ideas in a paragraph that most authors presented inside of a trilogy. So much of DnD's cosmology is derived from his books - why, when you kill a devil does it not actually die but reform on it's own plane? Because Gygax stole that idea from Moorcock:
As Oscar Wilde allegedly said "Imitation is the sincerest form of flattery that mediocrity can pay to greatness". Arneson and Gygax may have evolved the basic ideas around an RPG, by they proceeded to initially flesh it out with little regard to being inventive.
To be fair, creating detailed and consistent settings for fictional content is not a matter of a moments thought, so most great works of art or fiction build on prior art and history. The trick is to understand when you are overstepping the mark of plagiarism; Moorcock probably didn't care too much about his idea being reused, but TSR didn't get away with using Hobbits, Wargs and Balrogs for very long
