Another house rule we’ve recently adopted in the White Sandbox is that instead of affecting your armor class and thus your chance to be hit, each +1 worth of magical protection gives you a 1 in 10 saving throw against the effects of the hit.
Let’s say that Caswyn of Apollo is wearing one of the Gray Company’s namesake +1 cloaks of protection when he is struck by a medusa’s dagger. Caswyn’s player Eric calls out two numbers on a d20 and rolls it. If either of Eric’s lucky numbers come up, Caswyn’s cloak of protection has stopped the blow; he takes no damage and doesn’t have to save vs. the poison on the dagger. If Caswyn also has a +1 shield and +1 platemail in addition to his cloak, Eric gets to roll three d20s; of his lucky numbers come up on any of them, his gear saves him from harm.
The idea behind this house rule isn’t to change the statistical benefit of magical protection. Some rough analysis suggests that a 1-in-10 armor save helps you slightly more than a +1 to AC when you’re facing a high hit dice creature, and slightly less against a weaker enemy whose base chance to hit you is small. I see this as a nice side benefit, but the point is to change the way that magic protection feels. (For example, even if both are statistically equivalent, I think there is a very different feel in older editions when the target of a charm spell makes a saving throw and avoids its effects, versus in 4E when you make an attack roll for the spell and miss.)
A guiding principle for this house rule and the one about not rolling your hit points until you’re hurt is to take aspects of the game that normally get resolved off-screen beforehand and instead make them happen at the table as the spotlighted consequence of a dramatic event.
As the DM I roll to hit the PCs many times in an average session. When one of those blows would have landed if not for the protection of Fred the talking magic amulet, it’s obvious only to me. Even if I use this fact to narrate the monster’s miss, it doesn’t seem as real to the player as if I say “The minotaur’s axe slices you for six points of damage” and they get to respond “Not so fast, let’s see if Fred can save the day!” Owning a magic item, and being able to survive a lethal blow, should be remarkable. Highlighting these with a separate resolution step in play makes sure they get remarked upon.
It’s also extra work for me to have to figure out why a roll misses someone due to magic items. The armor save house rule unloads some work onto the players. One of the things I like about “three little books” OD&D is that AC can be directly translated into armor type. When we don’t use modifiers to AC from Dexterity or magic items, I simply track whether PCs are wearing leather, chain, or plate and then figure out AC from there. If AC is instead a complex composite of factors I have to remember both what a character’s AC is and also what armor they’re wearing, with all the other things that implies.
Another great thing about OD&D is that there’s a narrow range of AC. Even in magical plate and shield, a fighting man needs to worry about being hit by a lowly man-at-arms 20% of the time. I like this because even if a PC’s magical protection will stop a blow most of the time, I want to make the players sweat in the interval between when I announce the hit and when their magic save comes through for them! Also, when I have dozens of men-at-arms in the combat it’s much easier if I can just roll a handful of dice and count all the 17s or above, knowing that such rolls always have a chance of hitting any target.
Two final notes to put this house rule in the context of the original rules and in actual play. The text about magic armor and shields in Monsters and Treasure says that “Armor proper subtracts its bonus from the hit dice of the opponents of its wearer. If the shield’s bonus is greater than that of the armor there is a one-third chance that the blow will be caught by the shield, thus giving the additional subtraction.” Rather than try to reconcile the Arnesonian, Chainmail-based proto-D&D implications of that with the alternate-combat-system under which we normally roll, I’ll simply appropriate the idea of the probabilistic protection of a magic shield as support for the spirit of this house rule.
The need for these house rules came from a newfound prevalence of protective items in the White Sandbox campaign. This emerged because, until last session’s return to the Caverns of Thracia, we’d been looting nearby destinations in Jaquaysland, like the Fabled Garlin of Merlin (The Dungeoneer #2, editor) and Borshak’s Lair (The Dungeoneer #3, author), as well as one unique to the campaign, the workshop of the lich Patariki Van in the Nameless City. The consequent increase in bling was purposeful; I love the Caverns, but their distribution of loot doesn’t leave adventurers well prepared to face the stiff opposition in its deeper levels. (At EN World, Bullgrit has raised similar complaints about B1: In Search of the Unknown. Evidence of an anti-“Monty Haul” backlash taking place between 1976’s rich hauls and the spartan offerings of TSR and Judges’ Guild in 1978-9?)
EDIT: Maldoor’s comment below reminds me of another rationale behind these rules: players like to search for anything that’ll save their bacon when a PC’s life is on the line. In more rules-heavy games, the party goes modifier hunting: “Did you remember your temporary hit points? Did the monster hit even with its -2 to hit from bane?” Having a rule that kicks in at this tense moment will hopefully replace the urge to rules-lawyer and find ways to retcon it didn’t happen. Also, a game in which a character is never in danger because of their unhittable AC is boring; one in which that same character is quickly brought to the edge of death because they don’t remember to make their armor saves until it really counts is exciting!