I have only joined WoW about 1 week before the launch of WoTLK. I have always been a Warcraft fan however World of Warcraft had never caught me up strong enough in order to drive me in, the release of the Death Knight class changed that. My initial though when I joined was to level as quick as possible in order to reach level 55 and create a (not so) shinny Death Knight. I teamed up with a friend who started together with me and with the Refer-A-Friend bonus we ran by the old content. Unfortunately because of that I failed to see much of what the pre-TBC content had to offer. After reaching end game, I could only drive myself to level 2 more characters all the way through, and after experiencing the great leveling experience from Northrend, the old "vanilla" content always felt so boring and empty and so I couldn't find enough strengths to explore it as I should have. Long story short, I have never really experienced the pre-TBC content as I should have, with that in mind, the complete overhaul of the old areas and the leveling experience to me is probably the biggest reason it will make me buy Cataclysm. The Northrend leveling experience together with the old content is what I'm looking forward the most in this expansion, the opportunity to go back and start once again and make up for what I have missed when I foolishly rushed through all this content.
If I have to pick a second reason, I would say the Goblins, I wasn't too sure about them when they were firstly announced, Goblins never were my favorite races by far, however, after watching the clips from the beta and the new flood of info I couldn't disagree more with my first impression, they look by far one of the coolest races of the game and will probably be my favorite Horde race (replacing the Blood Elves). Not only that but their starting zone looks completely stunning and full of Lore which is what I love the most in this game! :)