Good question. First of all, there are a lot of definitions of "easy" leveling. Some classes are distinctly hard to kill and easy to play, making them great for beginners. There are other classes that do extremely high damage and can take on many enemies at once, making them very quick to level.
If you want to go for really fast leveling characters, I recommend high
DPS pet users like hunters and locks. Both classes are essentially
their own 2 man team, doing ranged damage while their pet keeps enemies
busy. This allows them to easily kill multiple enemies. They also have
a lot of little tricks to stay alive, making them ideal for massive
leveling. Of all the classes I have played, my lock and hunter were the
fastest and easiest to level. That's why of all my alts, they are the
ones that are 60+
Paladins are probably the easiest classes to play. At their simplest, you just run up and bash things down, and heal yourself when you get in trouble. Paladins are notoriously impossible to kill. A well geared paladin can slowly beat down many enemies at once. You can technically get by with hardly any skill and still level relatively well with these classes. The down side is Paladins are some of the slowest at leveling without very good gear.
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