There is no comprehensive list of rules yet, but the fine folks at MMO Mechanics have spaded a list of formulas to start off.
Player Life
Life when player level < 35 = 36 + 4 × Level + 10 × Vitality
Life when player level ≥ 35 = (level - 25) × Vitality
Damage
Damage on Character Sheet = (1 + passive skill boosts)*(Weapon DPS + ((minimum damage bonus + maximum damage bonus)/2)Attack Speed)(Weapon Damage Multipliers)(Attack Speed / Weapon Speed)(1 + ( crit% * crit damage %))*( 1 + (main stat / 100))
Spell Damage Modifier = (1 + passive skill boosts)(Weapon Damage + (minimum damage bonus + maximum damage bonus) / 2)(1 + (main stat / 100))
Defense
Total damage reduction = 1 - ((1 - DR from Armor) × (1 - DR from Resistance) × (1 - other DR))
Physical damage reduction = Armor / (50 × Monster Level + Armor)
Elemental damage reduction = Resistance / (5 × Monster Level + Resistance)
Offense
APS (attacks per second) = base weapon APS × (1 + dual wield bonus + other attack speed bonuses)
Experience
Monster level compared to character level XP gain
3 levels higher and above 125%
2 levels higher 120%
1 level higher 115%
Same level monster 100%
1 level lower 90%
2 levels lower 80%
3 levels lower 70%
4 levels lower 60%
5 levels lower 45%
6 levels lower 30% (no +XP/kill from gear)
7 levels lower 15% (no +XP/kill from gear)
8 levels lower 5% (no +XP/kill from gear)
9 levels lower 1% (no +XP/kill from gear)
Group Mechanics
Normal
# Players HP% Damage %
1 100% 100%
2 175% 100%
3 250% 100%
4 325% 100%
Nightmare
# Players HP% Damage %
1 100% 100%
2 185% 100%
3 270% 100%
4 355% 100%
Hell
# Players HP% Damage %
1 100% 100%
2 195% 100%
3 290% 100%
4 385% 100%
Inferno
# Players HP% Damage %
1 100% 100%
2 210% 100%
3 320% 100%
4 430% 100%