Intelligence

Intelligence is the ability to acquire and apply knowledge and skills. Intelligence grows the head, making the user a more powerful caster.

This is the Mage's, Priest's and Warlock's main stat. All classes can benefit from Intelligence due to the  Elemental Damage,  Mana, and  Resistance it provides.

Interestingly Intelligence also increases  Armor Penetration. Making it offensively useful for the Warrior and other  Physical Damage dealing Classes.

More Intelligence means a larger critical hit area, which is a significant drawback.