In my recent post about magic-users and armor, there was one option for doing strict class-based AC (that is, all fighters have AC 3, all magic-users have AC 9, and so forth). One downside of that system is that other than gear with special enchantments it doesn’t allow for much in the way of advancement (which is a big part of what makes D&D work as a game).
Here’s another idea that works with OD&D hit dice to address that concern. Take the combined hit dice value (for example, HD 5+1 = 6) and subtract that from 9 (or add it to 10 if using ascending AC). That is the characters armor class (improved further by one if using a shield). This gives fighters the best AC, but also allows gradual progression. Following the same example, a character with 5+1 HD has an AC of 3 . This is sort of the defensive equivalent of using hit dice as attack bonus. AC should also be capped, depending on the desired campaign power curve (if I was using this for Vaults of Pahvelorn, the best AC from hit dice would be 3, or 2 with a shield, because danger should always remain). Really, the term “AC” here becomes a bit vestigial; it’s really more of a defence stat, but continuing to call it AC probably helps from a UI perspective, given that it works exactly the same as AC.
If you wanted to preserve some mechanical effect from armor, maybe allow it to add a bonus to the death saving throw† (light = +1, medium = +2, and heavy = +3). This further reinforces the idea that HP is a mixture of elan and resolve, and that there are no potentially telling blows until HP have been exhausted. The downside of armor would be an encumbrance style penalty to ability checks, non-death saving throws, and escape rolls.
† Rather than deal with negative HP or have death occur at 0 HP automatically, I allow PCs to make a death save. If this is passed, the character is unconscious. If it is failed, the character is slain. This is one of my favorite house rules, and probably deserves its own post so that it can be linked to directly.