Unholy, Blood, or Frost for Death Knight Leveling In Cataclysm

Really depends on your favorite flavor of leveling on what spec you wish to use while hitting the new 85 level cap.  Currently I'm using a dual spec of Blood and Unholy, which allows me to tank the new dungeons and kill things faster while questing.  At this point, the cost for dual spec shouldn't be a hindrance and any hybrid class should be taking advantage of the ability.

But you say, 'I don't have good enough gear to run the new normal dungeons in Cataclysm as a tank.'  Pfsssh, the first few quest rewards are green items of ilvl 277 or higher, which will be the best gear available at lvl 80, pushing you above the arbitrary gear score Blizzard requires for queing in the dungeon finder.

Been doing a bunch of quests in Hyjal and starting to get burned out, take a break and go discover the new dungeon entrances.  As of this posting you cannot use dungeon finder to que for a dungeon until you discover it's entrance.  This means at level 80 you need to discover Blackrock Caverns and Throne of the Tides.

Fast XP

I will tell you that you need to take time to run each low 80 dungeon at least once.  The amount of XP received from running Blackrock Caverns is just a crazy amount and the new quest system is amazing.  The quest giver and receiver is all inline within the dungeon, so no returned trips necessary.  Kill 'X' and receive upwards of 70k experience and with all the bosses in Blackrock Caverns this adds up really quick.

Blackrock Caverns