Mythos/Statistics

Dexterity
Dexterity increases the amount of damage dealt with ranged weaponry as well as your character's total armor.

Strength
Strength increases the amount of damage dealt by melee weaponry.

Vitality
Vitality increases your character's maximum health.

Wisdom
Wisdom increases your character's maximum mana. Every 4 Wisdom increases spell damage by 1%.

Luck
According to Flagship, Luck helps calculate the drop percentage for higher quality items. In other words, Luck increases the quality of the drops, not the amount of drops.

According to TiberiusAudley of the Mythos forums, this improvement in quality is calculated with the following formula: $$( LUCK / 200) * ( LUCK / 200 ) * ( LUCK / 200 ) * 100$$

Due to this, the rewards of luck seem to increase exponentially (for example having 200 luck calculates a value 512 times larger than having 25 luck). But know however that the remaining pieces of the calculation process remain hidden - those regarding how items are dropped, the effect of luck on uniques (yellows), and what this drop quality percentage actually determines.