General conspiracy theories suggest WoW ripped Starcraft and Warcraft from both Warhammer franchises, which in turn ripped off all their stuff from a bazillion other fantasy/sci-fi sources.
While that's true, the stuff featured in 40k was pretty creative and relatively new at the time. Blizzard was more blatant in their "copping" of certain elements for Starcraft than they were in Warcraft, which featured a relatively standard fantasy world.