If you don't read a lot of articles and theories about game design you may not have heard the term "emergent gameplay" before. It's pretty much what it sounds like from the name - new ways of playing a game that emerge, rather than being immediately present. Often these new ways are unexpected by the people who made the game ... kind of like buying a toy for a child, only to discover the child spends the day playing with the wrapping, instead of the toy that YOU thought was the fun part.
www.jeuxvideo.com (cited in English in this Wikipedia article) defines it as follows:
Emergent gameplay is the creative use of a video game in ways unexpected by the game designer's original intent. It commonly appears as complex behaviors that emerge from the interaction of simple game mechanisms.
A simpler way of expressing it might be the old proverb:
You can lead a horse to water, but you can't make him drink.
Although game designers try to anticipate what players will do with their content, the more complex a game is, the more new combinations of the game elements players can come up with, often with surprising results.
In an MMO, which is complex from the start, AND is heavily based on interaction between players, emergent gameplay frequently makes its appearance, often to the startlement (and occasionally complete bemusement) of the game designers.
A couple of examples of emergent gameplay from EverQuest II:
1. "Lawn Darts"
Game element: the griffon transport system that allows players to swiftly ride on an airborne griffon across some of the world's larger zones, rather than travelling on foot. Players can ride to fixed destinations, or jump off mid-flight if they wish to stop at a location they fly over.
Designer intention: to shorten travel time between key points in the zone.
Emergent gameplay: "lawn darts". As far as I know this game was created for The Festival of Unity, a player-run roleplay event on one of the EQII servers. One player stands on the ground at some point under one of the normal griffon flight paths. Competitors catch the griffon that flies past him. The players jump off the griffon and attempt to land as close as possible to the target player; the closest player wins.
2. Book building
Game element: player-created books. With Game Update 52, EQII introduced player-created books. These are books whose text contents can be written by players themselves, and placed in the player's house or given to others.
Intention: To support and empower the players who like to tell stories, record their knowledge, and leave their mark on the world, emergent gameplay in itself.
Emergent gameplay: players are now proposing using the books to build forts inside their houses.
None of which has any particularly deep or meaningful point, but just goes to show that no matter how much game designers think they can anticipate what players want and how an addition to the game will be used, there are always unpredictable surprises. And how a game design team handles emergent gameplay -- whether they try to stop it, ignore it, or try to support it -- tells a lot about the character of the game and the philosophy of the design behind it.
Whichever the game, and however the designers choose to react to the emergent gameplay that the players come up with, I like to think that it has the potential to make the game more fun for both the designers and the players!