I am an unrepentant altaholic. In Wrath of the Lich King, I had seven level-capped characters. When Cataclysm came, I had some strange idea that I would work on those level 80 characters and get them to level 85. Instead of going that practical route, I'm currently in the middle of leveling both a warrior and a hunter ... both of which I already have at level 80. Cataclysm leveling is just that cool.
One of my favorite parts about the revamped old world is that the bosses of instances have been retuned for post-4.0 DPS levels. In previous expansions, boss fights would last mere seconds. This made it a horrible gauge for DPS, as burst specs would always win out and didn't help you learn your rotation. Now that the dungeons have been retuned and fights can last up to a minute, it is worth taking the time to take a look at your priority system for leveling.