Wrathgate and the Siege of Undercity.

Wrathgate - Before

Wrathgate - After

I love Blizzard's phasing tech. It makes the environment around you change as the storyline develops. Not everyone who runs into it sees it the same way you do (unless they progress the storyline the way you do). This was used extensively for the Death Knights starter quests.

Wrath Gate is one of the cooler questlines in Dragonblight, and the game in my opinion.(Heck it even comes with it's very own cut-scene) Blizzard also mentioned at Blizzcon that they will use phasing extensively in Cataclysm since they mastered the art of it in Wrath of the Lich King.

Also during the Siege of Undercity quest which occurs as part of the WrathGate chain, you get to invade Undercity with King Varian Wrynn and gain a blessing that increases your power ten-fold. You are pretty much invincible throughout the whole thing (as long as you stay with Wrynn and Jaina) - One thing I noticed in the beginning, your standing there with Broll, Wrynn and Valeera Sanguinar. For those of you who don't know, these three characters are the main ones in the World of Warcraft comic book which I collect.