Core Rules = Lost Carcosa, plus the "Little Brown Books," AKA Men & Magic, Monsters & Treasure, and The Underworld & Wilderness Adventures (plus the Reference Sheets)
Rule Clarification: The retroclone Full Metal Plate Mail, The Strategic Review, Chainmail, the "Holmes" Basic Set, Supplement I: Greyhawk, and The Dragon/Dragon/Dragon Magazine will likely be referenced. Others may be added if desired.
Experience and Levels
All classes follow the Full Metal Plate Mail version of my Unified XP Chart to determine how many experience points are needed to gain each level.
Level 13 = Maximum level for all PCs.
No training is needed to gain a level.
All PCs are humans or Ghouls. PCs cannot be Dwarves, Elves, Halflings, Zoogs, etc.
Humans can be either Fighters, Magic-Users, or Clerics. Ghouls are a class unto themselves.
All human PCs detect secret doors like Elves (1-4 on d6 for deliberate searches, 1-2 on d6 when passing by), detect traps/underground features like Dwarves (1-2 on d6), and detect sound through doors like Elves/Dwarves/Halflings (1-2 on d6).
All PCs begin play knowing one additional language of their choice beyond the default.
All spells in the "Little Brown Books" and Lost Carcosa are known by members of their respective classes as soon as they are at a high enough level to cast them. However, spells from other sources (e.g. the Supplements) must be either researched at the normal cost or found and copied from a scroll or spellbook before they can be added to a character's own spellbook, and thus memorized and cast. The DM reserves the right to veto the existence of any spell.
Most magic items are identified immediately. Potions must still be sipped to ID, though.
The new magic items of Lost Carcosa may be used by PCs of all classes.
(Something I'm still chewing over: How do Clerical scrolls work?)
Other General Changes
Ability Score rolls = best 3 out of 4d6, arrange to taste.
PCs are knocked unconscious at 0 HP, and die when reduced to their level in negative HP. (E.g. Level 4 = Dead at -4 HP)
Two-handed melee weapons add +1 to damage, except for staves wielded by Magic-Users.
Unarmed attacks do d2 points of damage. Shoddy and improvised weapons do d4 damage.
Dungeon doors are not stuck closed by default, but specific ones may be stuck as chosen by the DM.
Fighters (AKA Fighting-Men, as I'm using these terms interchangeably)
Strength Bonus: Fighters get +1 to hit with melee attacks and do +1 melee damage if their Strength is 15 or greater.
Multiple (Normal) Attacks: Fighters can make 2 normal attacks per round starting at Level 4 (Hero), 3 normal attacks per round starting at Level 8 (Superhero), and 4 normal attacks per round starting at Level 12.
Crowd Control Attacks: Instead of attacking normally (i.e. in the above fashion), Fighters can make one melee attack per each of their HD per round against enemies of 1 HD or less. Fighters cannot make multiple attacks in this fashion (i.e. "Crowd Control") and in the above fashion (i.e. multiple "normal" attacks) in the same combat round.
Dwarf Weapons: Human Fighters can use magic weapons normally reserved for Dwarves.
Intelligence Bonus: Magic-Users get 1 extra First Level spell per day if their Intelligence is 15 or greater.
No "Read Magic" Spell: Read Magic is removed from the spell list. Scrolls, runes on wands and magic items, and other typical magical writing that normally requires the use of this spell can instead by read automatically by any Magic-User. This skill is considered a basic part of a Magic-User's training. Essentially, every Magic-User is under the permanent effect of a "Read Magic" spell at all times.
Weapons: Magic-Users can attack with staves as well as daggers.
Wisdom Bonus: Clerics get 1 extra First Level spell per day if their Wisdom is 15 or greater.
Level 6 Spells: Clerics gain one Sixth Level Spell per day at Level 12 in addition to their normal spells per day in order to be able to cast the new Sixth Level spell in Lost Carcosa. (If the maximum of Level 13 is ignored, see Supplement I: Greyhawk for higher level spell progression.)
Dwarf Weapons: Human Clerics can use blunt magic weapons normally reserves for Dwarves.
Spellbooks and Research: Clerics use spellbooks like Magic-Users. Clerics can research new Clerical spells just as Magic-Users can research their own spells, and the cost of doing so is calculated the same way.