The engine may be old, but the game actually looks amazing, especially in the Draenor zone. After the characters model update, it looks better than Wildstar, which is another cartoony MMO, and it requires much lesser hardware to run smooth.
It might not have the photo-realistic graphics of GW2 or ESO, but those games feel bland, they lack "soul". The zones in WoW feel charming, cozy, magical.