I've always felt armor should be kept in 4 some-what abstract categories/tiers, and then have the material an/or quality dictate individual armor pieces exact stats, with armor types/designs relating to appearence. The tiers really just determines how the specific piece of armor relates to, protection, mobility/speed and endurance. Protection could be, deflecting attacks, reducing damage, or even increasing an opponents miss chance (a sheet of clothing may not stop a sword, but it makes you difficult to see!). Mobility could be your ability to outright dodge, use skills, how quickly you traverse the battlefield, or how quickly you attack. And Endurance would dictate how readily your skills and abilities are available to you, this might increase the cost of a skill, the time it takes between uses or weaken the an abilities effectiveness.
The 4 tiers are the established standards:
No armor / Adorments (Tier 0)
Protection - None (_)
Mobility - 100% (+++)
Endurance - 100% (+++)
Light (Tier 1)
Protection - Minor (+)
Mobility - 100% (+++)
Endurance - 110% (++)
Medium (Tier 2)
Protection - Moderate (++)
Mobility - 90% (++)
Endurance - 110% (++)
Heavy (Tier 3)
Protection - High (+++)
Mobility - 90% (++)
Endurance - 120% (+)
Once a piece of armor falls into a category, it's material and quality would further adjust the above basic stats or add new qualities to the item.
Materials:
Iron - Increases Protection (+), Decreases Mobility (-), Decreases Endurance (-), Vulnerable to Rust
Mithril - Increases Protection (+), Reduced Weight, Penalty to Stealth
Qualities:
Masterwork - Increase Mobility and/or Endurance (+)
Full Suit - Increase Protection (+), Reduced chance to be critically hit
So some examples of armors would be as follows:
Breastplate (Heavy, Iron)
Full Plate Armor (Heavy, Iron, Full Suit)
Leather Cuirass (Light, Leather)
Leather Armor (Light, Leather, Full Suit)
You could make armor designs, such as plate, scale, chain and make them armor qualities, but I'd personally rather make them dictate appearence. So mechanically scale armor (heavy, iron) and plate armor (heavy, iron) would be the same, but would have distinct appearences. Basically you're decoupling the armor design (plate, scale, chain, etc.) from the stats of the armor, and instead providing the stats through the various tags (heavy, iron, masterwork, etc).
It's a complex system I know, but I've always preferred to start with high complexity and then trim down and streamline a concept. I just hope it makes sense hehe!