I dual boxed all of Hyjal, did a bit of Vash'yr (I found the underwater a bit confusing though) and have just started Deepholm once I hit 82.

My experince of cata has been awesome, flying into Hyjal and Deepholm is truly breathtaking and really smashes in the scale of these areas.

That being said there are only 2 quests I have hated so far, these are:

http://www.wowhead.com/quest=25599

On launch night this was impossible to complete as the boss drops a chest that only 1 person could open, and the boss was on a 2-3 minute respawn timer. Awesome world pvp was happening in here though, I ganked many hordies trying to sneak in and ninja the chest lol.

http://www.wowhead.com/quest=25525
Blizzard managed to make something as awesome as aerial jousting into one of the most awful vehicle quests ever. Easy to mb though as you can leave your slaves at the quest giver while you do the quest on your master and all get credit.

This quest seriously pissed me off, aerial jousting could be all types of awesome if we had complete control of the mounts, seriously flying is awesome, why break it with a stupid button to 'flap'???