Burning Crusade had demons. Wrath of the Lich King had undead. The fact both expansions had a clear and definitive theme with their mobs was extremely convenient for alot of classes and builds. Paladins, at least, had a myriad of mechanics especially effective against demons, and I'm almost sure there was some time of demonslaying dps enchantment that was actually viable in TBC, because of how great your odds were of having to fight demons. Wrath was even more one-sided, as almost EVERY freaking raid and zone was undead centered. Again, paladins shined in this expansion, and I know for sure there was a scourgeslaying enchantment that was very relevent to dpers' interests.
TL;DR: The last two expansions featured a definitive NPC type extremely convenient to certain classes and builds. Do you think Cataclysm will follow this pattern? If so, with what NPC type? If not, why?