Earlier quoted context omitted.
Just to emphasize your point in the last paragraph: comparing the game mechanics in Zynga games to those in traditional MMOs is laughable. Zynga games are more comparable to social websites than WoW.
Could you point out a WoW mechanic that is actually taxing? From what I understand, WoW AI is a height agnostic beeline and battles are heavily scripted at best. The tricky part seems to be the real-time player interaction a.k.a 3d chatroom, which has nothing to do with game mechanics (e.g., IMVU).
Movement is one already mentioned, but in general, AI isn't only turned on in combat. NPCs move outside of combat - most creatures in the world wander about or follow a pre-defined path. I don't recall if they did on WoW, but I know in Everquest creatures used spells of their class both in and out of combat, such as buffs and heals - a creature in Everquest would even heal and buff up their allies. If monsters are buffing, you now are tracking those buffs & their durations - when we added monsters buffing to our multiplayer game, we made them infinite in duration to get acceptable performance.
Another operation that could be taxing is calculating line of sight (LOS) - this factors into any action an NPC or player would do. You can't have monsters attacking players, or buffing allies, through walls.