I guess it all depends on what amount of the WoW setting you're going to specifically use. I mean, would it just strike as Ye Olde Generic Medieval Fantasy world, if you didn't mention any of the WoW characters?

I don't know much about the WoW fandom, but I guess...if a WoW fan would ping on your stuff as being WoW fannish without being told, then it is fanfic and not origfic. Use your own judgment.

Now if you took your characters and put them into another fantasy world that was wholly your own creation (or, let's be honest, one where you'd changed enough of the details that no one could TELL that it was still WoW in your mind), then that'd work. ;P