New behaviors that evolve organically beyond lower-level characteristics.
In game design, emergent behavior can be your best friend or your worst nightmare. Players love quirky emergent behavior because it is unexpected and cool to see systems interacting in unintended ways. Emergent behavior can often be encouraged through complexity; creating more complex systems and mechanics or giving players more options to interact with the world. Think sandbox games.
MMOs often struggle with the negative effects of emergent behavior. Many MMOs are designed around cooperative gameplay, but getting randoms to cooperate together is hard. More often than not, MMOs devolve into uninspired slugfests that replace intended gameplay. The reason why this so often occurs is a lack of correct incentives and punishments. You have to be very careful how you design a multiplayer game for cooperation. How do you reward cooperative play? How do you punish team killing, sabotage, and sandbagging? If you don’t have ready incentives and punishments for these things, you are on the game design path to hell. Change course…quickly.