Well, Warcraft started as medieval fantasy and has stayed that way from Warcraft 1 to 3 and even closely stuck to that in World of Warcraft Vanilla. It's only with the Burning Crusade that they started to heavily experiment with all the steampunk, sci-fi punk and whatever else people would want to call it.
Even the expansion of Warcraft III touched upon a little bit of sci-fi stuff, but still closely stuck to a medieval fantasy theme even in Outlands.
Wrath of the Lich King went back to basics by sticking to a more heavy medieval fantasy theme overall. Cataclysm was their attempt at trying to bring an expansion with heavy Lovecraftian influences. And now Mists of Pandaria... they just try to bring us an ancient asian theme.
But honestly. I think people love Warcraft best of all when it sticks to medieval fantasy.
I personally just had to shrug my shoulders when I saw them bringing in LOLSpaceships, Steampunk Naga and Lazorbeam Blood Elves or Industrial Demons in the Burning Crusade.
How people can say that Warcraft armor or graphics look better than NWO goes far beyond me. The graphics and attention to detail in NWO made my jaw drop quite often. While WoW ... well... it reminds me very hard that I'm playing a game made in early 2000. The graphics look very similar to some RPG games for on my iPhone.