This is an absurdly tough question to answer.
Of all the things coming in Cataclysm, to pick one and say it's THE thing you're looking forward to most seems impossible. I think, for me, It's just being able to see a bigger and more fantastic Azeroth. One of the things about this franchise that has always struck me, was it's ability to just be... wonderful. Wonderful visually, I mean. The art and landscapes of the world are just completely stunning, and what I've seen in Cataclysm, it's only getting better.
I'm a firm believer that video games are art, if not one of the more capable forms of art. Blizzard has always done a stand up job of proving this point, it becomes childs play to convince others the vast capabilities of video games as art when you have examples like WoW. One of the biggest challenges of the industry is making an engaging story that a player can still partake in, especially with mmorpgs; If you go to any cataclysm forum, you'll find people going completely nuts over the new story developments in this expansion. You'll see examples of quests that read like novels and little scenes between npcs that feel like you're watching a movie. I think the player base underestimates how incredibly amazing that really is.
Take for example, the incredible amount of Garrosh threads there are. People either love him or hate him, and the community gets REALLY worked up about the subject, almost all these threads turn into heated debates (or flame wars :P). People genuinely care about this universe. I may have gotten a bit off topic, but it does contribute to my main point.
What I'm looking forward to most about Cataclysm is everything that follows it. I'm looking forward to this beautiful world evolving and becoming even more fantastic and immersive. I'm invested in the story of this universe, and I wait eagerly to hear the next part of the tale. That's why I raid (that and shiny mounts), I want to be right there when story points happen. Warcraft is something more than a video game, and each expansion solidifies this notion. The world in World of Warcraft is making it's case and telling everyone "YES. I BELONG HERE."
Perhaps it's too vague or broad of a "one thing" to count, but it's my answer. I know I'll be playing this game for a long long long long long time, and cataclysm is only proving that my time will be well spent in the future.
Thanks for reading my long winded post :)