I have been considering the idea of using an alternate system for armor in D&D-like games. Instead of increasing the character's Armor Class, the armor would reduce the die type of an incoming attack. Depending on the dice chain that a GM uses this could be marginally deadly or make damage virtually nonexistant. This means combat would also be bloodier as everybody would be getting hit more often, but warriors and clerics would take less damage respectively since they are typically more armored than rogues and wizards.
The way those are read is Leather reduces incoming damage by one die type, Chain armor reduces damage by two die types, and Plate armor reduces damage by three die types. A Shield would reduce damage only by -1 point. Reducing a d4 would simply add a -2 to the total damage inflicted and damage could never be reduced lower than 1 point, so a successful strike will always deal at least 1 damage.
This is something I've toyed with because I've always wanted to create a system where every PC starts with the same amount of Hit Points (20) and very little variation exists between classes. Monsters would also have a similar amount of Hit Points, but could get increases due to size or supernatural traits.