From my experience ...

Hunter: so easy a gold farmer can do it. Very powerful, lots of crafty tricks you can learn to make it easier/faster/better. Can be a terrific DPS class at 80. You must learn to control your pet.

Warlock: almost as easy as a hunter. More powerful in the 65+ range. Leveling as SL/Fel Guard I was able to grind non-stop to 80, and could solo all group quests (excepting the arenas) all the way to 80. Can be a terrific DPS class at 80. You must learn to control your pet.

Druid: Starts slow. Can heal yourself. Becomes a lot of fun when you get cat form at 20. Really gets going around level 30 and is a breeze (as feral) after that. Can be a tank, healer, spell casting or melee DPS at 80. More options than any other class.

Paladin: dreadfully slow until 20. Seriously, until 20, you have auto-attack, and judgment. That is your entire offensive arsenal. At 20, you get some additional toys to play with, and it starts to become easy. Self healing, immunity bubbles, mana and health regen abilities make it a great class to solo with. Can be a tank, healer, or melee DPS at 80. Lots of options.

Mage: starts slow. Lots of downtime to drink, but you make your own food/water. Great crowd control/AoE abilities. Frost AoE builds can do some serious AoE grinding at times, though nowhere nearly as good as it used to be. Can be terrific DPS at 80. Plus, you bring free food/water and a portal home. Everybody loves you for that :)

Death Knight: Lots of fun to play. Blizzard did a great job with the starting area introducing you to your abilities a little at a time. All three trees are viable to leveling. Personally using Unholy for its wicked DPS. Can tank or DPS at 80, but everybody and their sister rolled one.

Warrior, Rogue, Shammy and priest I never made ti past 35ish.