It is hard not to recognize the similarity in races and the cartoonish graphical style of Warhammer Online and World of Warcraft and not compare the two.
However, while World of Warcraft was released years before Warhammer Online, it is actually more of a clone of Warhammer than Warhammer is a clone of World of Warcraft.
The Warhammer pen and paper roleplaying game was the inspiration of the original Warcraft realtime strategy game. Not only did the Warhammer RPG influence the basic Good vs Evil structure of the game, it also had a large influence on the graphical style of the Warcraft series.