WoW used to run update only twice a second, which gives a lot of time for logic. The WoW game logic is very simple, as the players cant really affect the world other than the enemies they are fighting at that moment. Also, the players are spread around the world and that limits the N^2 problem of players sending data to other players. Events like Gates of Anquiraj made servers break due to clumping too many players in one location. The game also used to do a lot of logic on the clients, for example collision checks would only run in clients, allowing easy flight-hacks. The server really only broadcasts player positions to other players, and manages the combat logic which is basically turn based.

In private servers using code you can look at (Mangos) they manage to run 4000+ players on a single gaming-spec machine without much trouble. Its more of a design problem compared to something like minecraft where the simulation is much more detailed. In some research projects for wow private servers people have reached 20.000 simulated players in a given machine.