Vitality: Each point of vitality increases your maximum HP by (level + 8) x 2.
Dexterity: Each point of dexterity improves your attack and defense ratings, and increases ranged damage.
Strength: Each point of strength improves your carrying capacity by 3 (up to a maximum of 500), and increases melee damage.
Willpower: Each point of willpower increases your maximum mana by (level + 8).