Primary

 Intelligence
 Strength
 Agility

Damage

 Physical Damage
 Elemental Damage
 Melee Damage
 Ranged Damage
 Fire Damage
 Ice Damage
 Light Damage
 Critical Damage
 Armor Penetration

Defensive

 Health
 Mana
 Armor
 Resistance
 Dodge
 Tenacity
 Life Steal
 Mana Steal
 Spell Vamp

Miscellaneous

 Move Speed
 Attack Speed
 Projectile Speed
 Cast Rate
 Jump Force
 Range
 Melee Range

Armor Penetration

Armor Penetration reduces the  Armor of a target. Armor Penetration is useful for Classes that utilize  Physical Damage, such as the Warrior, Hunter, and Rogue

Armor Penetration is not as effective for targets which lack  Armor and therefore have nothing to penetrate.