The Enigma Order

What I noticed that set wrath of the lich king apart from previous versions was something blizzard named Phasing quests.
This new concept is what I like the most about this expansion. Here is a few notes on the subject:

Phasing is a world design technique introduced in the Wrath of the Lich King expansion.

The process allows certain players to view certain phases of the world environment depending on what place they are in certain quest lines. Most often changes in world phase are triggered by turning in certain quests.

For example, in the Death Knight Starting Zone of Ebon Hold, players move through three different world phases while completing the Starting Experience's epic quest chain. After completing certain quests, the player returns to the area of Death's Breach to find it completely different from how it looked during previous quests: NPCs have changed position, certain buildings are now destroyed, etc.

Only players that are in the same phase of the world can see each other. If a player currently completing quests in phase 1 of an area is standing next to a player who is involved in phase 2 or 3, they cannot see or /speak to each other, though that can still communicate using private tells and global chat channels.

At times, phasing only effects an area's mobs. This can, at times, make it seem that players are fighting thin air.

You can find more information at: http://www.wotlkwiki.info/index.php/Phasing