The #1 thing I am looking forward to the most with Cataclysm is all the new lore. I have read all the books and played all the games, and to finally be able to go toe-to-toe with Deathwing himself is epic. To see the return of Azshara, the Queen of the Highborne herself! And not to mention the return of Malfurion Stormrage, back from his long slumber in the Emerald Nightmare defending Azeroth from the Nightmare. I absolutely cannot wait to meet with all these huge figures of lore, whether it's to fight against them in an epic battle, or to stand by their side while taking down foes.
And then to venture into unknown lands of lore as well. Uldum may have been around, but to see what Uldum REALLY is and what is inside will truly be a sight. And to dive into the depths of Grim Batol where so many wars and battles have raged over, where the Life-Binder herself was held captive, only to be freed later by a band of heroes.. And not to mention being able to visit Deepholm! Where Deathwing himself was nursed back to health after his defeat at Grim Batol, only to then enter Azeroth through the World Pillar, shattering it, and causing the Cataclysm itself.
And last, but not least, to meet the newcomers to the Alliance and the Horde. I have wondered for YEARS now what has been going on behind the Greymane Wall in Silverpine Forest.. Only to find out that Genn Greymane himself has survived only to be cursed with his people, now known to be Worgens. And to watch as Thrall is captured by SI:7 agents, and then freed by a cartel of Goblins, therefore unifying them within the Horde, will also be amazing to see.
These are the things that I look forward to the most.
(And flying in Azeroth comes in at second place too :) )