Ruleof Law changes the rules of how many spells can be played per turn. Furnace of Rath changes the rules on how much damage your spells do. Rites of Flourishing changes the rules of how many lands players can play and how many cards they draw per turn. The point is, tons of enchantments establish new rules for a game of Magic.

Forsaken Wastes

One type of global enchantments establishes a new rule that supersedes the last one. Printed mostly in Legends and Mirage, with sprinklings in Visions, Homelands, and Alliances, “Enchant worlds” were enchantments with a rule-setting flavor with a special rule that only one could be in play at a time. Now called world enchantments (enchantments with the supertype “world”), they interact via a “world rule*” that’s similar to the “legend rule.” When a new world enchantment comes into play, any other world enchantment(s) go away. When The Abyss lands on the table, for example, it not only creates a nasty creature-hostile environment; it also replaces Concordant Crossroads or Gravity Sphere or any other world enchantments that were in effect before. The feeling is, “Those were the old rules, folks—here are the new rules we’re playing by.”

Concordant Crossroads
The Abyss

Sometimes everybody’s happy to see the next world enchantment.
Sometimes everybody—except that creatureless black mage—groans.

The world supertype was dropped mostly because it didn’t add much gameplay value to balance the rules complexity it added. It is, after all, usually a drawback to have your enchantments have a built-in destruction clause, and most of what world enchantments were designed to accomplish can be realized with a more straightforward global enchantment.

Note that legendary enchantments, which have some of the same kind of gameplay as world enchantments, appeared in Kamigawa Block. Some powerful Kamigawa enchantments, such as Night of Souls' Betrayal or the Hondens, were made legendary to prevent the situation of having multiples in play. Some were legendary more for flavor reasons, like Genju of the Realm—it’s the one uber-kami that represents the spirit of all five types of basic land.

*The “world rule” from the Comprehensive Rules

420.5i If two or more permanents have the supertype world, all except the one that has been a permanent with the world supertype in play for the shortest amount of time are put into their owners’ graveyards. In the event of a tie for the shortest amount of time, all are put into their owners’ graveyards. This is called the “world rule.”