The thing to understand is that MMORPGs create the illusion that they're kind of like browsers -- all the content is on the server, the client is little more than a window into a shared world.

That's not the case. The bandwidth would be crippling if it were. Most of the game is in the client. Clients know how to do actions, cast spells, count cooldowns, etc. The server has some maps that tell the clients "put this immobile resource here, spawn this mob there" (but the clients know what those resources and mobs look and act like, the server just tells them to instantiate a mob of this id there), and then sanity-checks and synchronizes the clients.