Strength:
Increases weapon damage. Weapons and armours have Str requirements.

Constitution:
Increases hit-points and stamina. Armours have Con requirements.

Agility:
Increases chance to hit with weapons.

Reflexes:
Decreases the chance of getting hit.

Mind:
Increases mana. The manual also claims that "you will gain an edge in spell casting", whatever that means.

Luck:
Slightly increases the chance of succeeding with any task.

yes, boosts damage by 30% at Strength 30 +30% hp at stat 30 +30 to "to hit" possibility +30 to dodge +30% mana bonus +15 to roll modifier... I think the best value for this is 23: gives +10 to roll modifier

Yea if you want to wear the heavier armors you want your strength and con up. Even the basic sets start at

tier 1 i think is 16/14 and tier 2 is 18/16, so if you want to wear heavier armors at all you need to get strength and con up as soon as possible. Also heavier armors give you agility penalties (ability to hit foes) so you'll want to raise up agility at the same time so you don't lose your ability to hit stuff while tanking up with armor.

So with a warrior those 3 are must haves, strength, con and agility that go along with wearing heavy armor. If you just want to chuck meteor swarms out one after the other you won't need that and just up your mana/reflexes.