The question sparks an interesting discussion which I encourage you to read. It certainly makes me think about WoW in a new light. Suppose we imagine that Blizzard had made a different decision about which franchise to develop into their first MMO: would World of Starcraft have been as popular as World of Warcraft is today if its gameplay was more or less just as good, or is the science fiction environment inherently less appealing to the masses than fantasy?
To me, it makes sense that Blizzard chose the Warcraft franchise over Starcraft (or even Diablo for that matter), not just because it is fantasy, and certainly not just because it draws on elements of Tolkien's literature, but because it draws on a wider range of mythic elements, contains more layers of conflict, and generally provides a greater diversity of opportunities for a new gamer to find something he or she likes in it. What's your opinion?