*AKA Red Moose D&D**.
**AKA D&D Red Box+S&W White Box+Pathfinder+D&D 4E
1. The Fates
The DM chooses one of the two methods:
A) Roll 2d6. Take the lower result and subtract it from the higher. Then subtract 1 from that result. Do this five more times, once each for Strenght, Constitution, Dexterity, Intelligence, Wisdom, Charisma. That's your Ability modifier.
To determine your Ability, multiply your modifier by 2 and add 10. If you rolled an even result on your high die, add 1 to your Ability, unless it's already 18.
B) Alternatively roll 3d6 in order.
OPTIONAL: If you're using method B and want race to matter, use this rule: If you're a Dwarf, roll 4d6 drop lowest for Constitution and Strength. If you're an Elf, roll 4d6 drop lowest for Intelligence and Charisma. If you're a Halfling, roll 4d6 drop lowest for Dexterity and Wisdom. Humans add 1d6 to any two rolls and drop lowest in those rolls.
2. The Luck we Make
Pick a class: Fighter, Mage, Rogue, Cleric.
3. Ruling in Blood
Roll a d20+appropriate Stat Modifier. STR for physical action, DEX for skill, CON for endurance, INT for knowledge, WIS for awareness, CHA for communication and command.
The DM should set a Difficulty upfront (rule of thumb: 10 very easy, 12 easy, 14 moderate, 16 hard). The first pass is for initiating it. Difficulty +2 is for doing it. Difficulty+2+5 is for seeing it through.
Like: If a player wants to push the cabinet in front of the door, the DM says: "You'll need a 12+ to move it, a 14+ to get it halfway accross the room and a 19+ to push it all the way to the other end."
Failures result in complications. There are basically two types of complications: a) some new problem or threat appears or the first one becomes worse or b) you simply cannot do it now/this way, unless the circumstances change/you have to do something else first.
In the face of violence resolve first any attempts to flee or parley. When two or more parties enter combat, they should all roll a die (d20 is preferable) to see who seizes the initiative. People who begun the violence get a +4 bonus on the first turn. The player with the highest result declares first, then anyone who is directly impacted by their action can declare what they're doing in response. Then go on to the next highest initiative number, unless they have already declared and so on untill everyone has declared (either on their turn or in response). Then resolve the actions. They happen simultaneously, but resolve first for people with higher initiative.
Attacks rolls with melee weapons are d20+STR+Combat Bonus, ranged attacks are d20+DEX+Combat Bonus vs. 10+Combat Bonus+DEX.
If an attack is successful, damage is 1d4 for unarmed, 1d6 for small weapons, daggers and such, 1d8 for light, like short swords and clubs, 1d10 for serious weapons like swords & spears, 1d12 for heavy weapons like warhammers & polearms. Damage dice explode (are rolled again on a highest result).
Depending on the armour worn, the damage die type is knocked down by one or more levels: leather 1-, chain 2-, plate 3-. So a person in full plate getting hit by a polearm would only suffer 1d6 damage and would need to be hit by at least a serious weapon to even suffer any harm.
Optionally, passing the Defense by 2 or 7 (+2/+5) allows the attacker to increase the die type by 1. (Allowing even for light weapons to deal damage to dudes in full plate.) Either that or go for a grapple, disarming, knocking off the helmet whatever. Combat is descriptive.
Spells are d20+INT+Spell Level for Mages, d20+WIS+Spell Level for Clerics vs. apropriate Resistance (as determined by the spell). Resistances are 12+CON/DEX/WIS (for Fortitude, Reflex, Will).
Use the Combat Bonus + STR or DEX (whichever is more appropriate) +d20 for any combat maneuvers such as grapples, disarms, etc.
Helping requires a roll and gives a +2 for each of the three steps of success.
Attempts to flee or parley can be announced again at any time after a current round has been resolved.
I think that is almost everything.