Anyways, I was going to reply to the great comments in the "D&D for $2.99" post with my own ideas, but they started getting a little too involved for a tiny comment, so I'll elaborate here.
My own thoughts are similar to bits from each of the people who commented (Michael, Martin, and Geoffrey.) Plus, bits from elsewhere.
Format: This one's a big question. Part of the answer depends on who your audience is, and whether you're doing this for profit, as advertising, or to proselytize. Selling to gamers at conventions? You can get by on about a 16-page book, maybe comic book size (the RPGNet thread brought up the cheapness of this format.) Or maybe a tabloid. Selling to savvy geeks? Aim for some kind of app or small PDF people could read on their phones. Trying to bring in non-gamers? You might need a radical rewrite to condense the essence of the game to brochure form. Contrary to RPGNet, dice are not included.
Art: marginalia-style art, if any; that seems to be pretty much agreed-upon. Art would have to be donated to such a cause, or done by the publisher.
Characters: two classes, no demi-humans. Fighters get more hit points and better attacks, and can use magic weapons/armor and potions. M-Us get potions, scrolls, wands, and other non-combat magic items; they get no attack bonus on weapons other than daggers (this sidesteps the usual questions about M-U weapon/armor restrictions.) They know all the 1st level spells (our list will be shorter) and can cast 1 spell per level of each spell level known, up to 4/level. No alignments (assumed to be neutral.) Include a note that characters have average strength, intelligence, agility, or other abilities, but that more advanced versions of the game have players roll for their abilities.
Experience: much lower level cap, say Level 6. Give XP totals for Levels 3 and 6 (Fighter: 4,000/32,000; M-U: 5,000/40,000.) When you get half a given XP total, you are at (Level-1), and at double that total, (Level+1). Earn 100 XP/hit die or 1 XP/1 GP. Each level earns 1 hit die, half a hit die for M-Us; a "half-hit die" equals 1 hit point, trade in for a full hit die next level.
Weapons: d6 weapons, no differentiation except length and material. Clubs and walking sticks are free, other wooden weapons are 1 GP. Metal weapons cost double their length, in GP, or 5xlength for missile weapons.
Armor: The standard three. Unarmored people have Move 12; if you buy a shield (10 GP,) AC is 8. Each 15 gp spent on armor reduces AC by 2 and Move by 3, minimum Move 6. Not using a shield adds 1 to AC.
Equipment: no lists, but instead a stripped-down version of Zak S's penny, nickel, dime, quarter, dollar system. I'd condense his system to:
- 1 GP will buy stuff a typical shopkeeper might buy in a given day.
- 1 SP will buy lower-quality stuff that laborer might buy.
- 1 CP will buy the cheapest, commonest items, like watery ale or a bowl of gruel.
- Items that aren't day-to-day cost 5-10 GP per word in the name. Luxury or other out-of-the-ordinary items cost double or triple this, or more.
Dice: you need ordinary dice + digital watch with seconds feature. If the rules call for a d10, read the last digit of the seconds; if they call for a d20, roll a d6 and add 10 to the last digit of the seconds if the roll = 4 to 6.
Attacks: add target's AC + attacker's hit dice to d20 result (basically, this is Target 20.) Longest weapon goes first on first attack, last on subsequent attacks.
GM notes: If characters try to improve a bad situation, it works on a 2 or less (d6). If something can go wrong, it does on a 6 (d6.) Describe basic exploration mode and things like surprise, wandering monsters, and the need for light in a dungeon. Include the briefest notes on how to build monsters (plus a short sample list) and spells.
Additional Resources: where to find Labyrinth Lord and Swords & Wizardry, if you want a more elaborate version of the same game. Perhaps some monster, spell, and treasure links, too.