WoW has been programmed to use DX9.
Generally speaking, if you were to play WoW on a DX9, DX10, DX10.1 or DX11 card, the output will all look the same.
It might look different if you were to compare ATI and nVidia because of different drivers that handles graphic settings differently. I recall reading something way back when Oblivion was released where ATI drivers (I might need to chug a bottle of Macallan 12yr old Scotch to jog my memory) gave all NPC a "5 O'Clock shadow" based on how the shader algorthims were programmed. That was later corrected.