Vanilla - Everything was brand new and I was super excited when I was heading to a new zone and when I did my first instance (deadmines)
TBC - The look of outlands, 10 man ZA bear mount run, the story,Raids, Isle of Qual'danas, SSC/TK Hand of A'dal quests, Sunwell
Wotlk - Ulduar, Arthas, questing wasn't to bad either and you had two starting zones.
Cata - transmog, redone old world with better questing(still miss the old world)
MoP - Haven't been able to play yet...its gonna be awhile to I think...=(