WoW: WotLK - Review @ Gamespot

Gamespot like Blizzard's phasing technique in WotLK 9/10.

New in Wrath of the Lich King are numerous quests in which you complete objectives at the controls of a mount or vehicle. Quest-specific rides include dragons, mammoths, airplanes, bipedal mechs, and even a giant. The controls are slightly different for all of them, but they're never complicated and they're always clearly displayed onscreen as soon as you climb aboard. Another gameplay mechanic that's used frequently in new quests is phasing, which lets you see areas of the world differently from other players. Similar to how the world appears in gray scale when you die and have to run back to your corpse as a ghost, phasing quests often apply some kind of visual filter to the environment and let you interact with NPCs in different ways. Wrath of the Lich King takes this idea to a whole new level. By completing certain quests, you trigger dramatic changes to the environment that are the same for all players who have completed said quest, but for players who have yet to do so, the world still exists in its original form. Whether this is achieved though technical wizardry or just straight-up magic is unclear, but its integration is seamless, and it's incredibly satisfying to feel like your actions are having a significant impact on the world around you.