In reality, there are hundreds of different kinds of armor.
But in the same reality, there really are only two different kinds of armor.
Here's what I mean. As far as fluffy details go (chainmail vs. plate vs. crushed conch shells vs. hide vs. sewn leather straps vs. whatever the hell you've put all over your body to wade into battle with) warriors and rogues and such are looking to do one of two things: have a lot of protection at the cost of being encumbered, or have a little protection at the cost of only slight encumbrance.
That's it. Medium armor? It doesn't exist. Who wears medium armor? It's too encumbering for fast fighters and thieves, and not protective enough for front-line warriors. It's the equivalent of having a sword between the great sword and longsword.
Bastard sword, you say? See my opinions on THAT nonsense here.
I once tried to establish a piecemeal armor system for one of my games, and it was fun in theory, but impossible in practice. Without sky-high armor bonuses as the standard (like, +9, +10, +11 etc.), the piecemeal stuff was always overpowered, so I gave it penalties unless you took two feats to cancel those penalties, but that didn't make sense because why would you specialize your character in something that is the opposite of a specialty? The whole point of piecemeal armor was that you had to work with what you could find. Obviously, it went no where....
So the system I use goes like this:
Light armor adds a small AC bonus, and takes up a little inventory space, and causes a small stealth penalty. Light armor could mean a cool trench coat, a chain mail shirt, a suit of studded leather, or whatever.
Heavy armor adds a large (2x light) AC bonus, and has an equivalent inventory cost and stealth penalty. Heavy armor could be a breastplate with greaves and bracers, a full set of plate armor, etc.
Then I add shields (+1 AC) along with "Shields Shall Be Splintered!" At the cost of 1-hand in combat, I find this to be very balanced.
You can also wear a helmet for an additional +1 AC and protection from head-based critical hits/fumbles/death saving throws. Since head strikes are so uncommon, and because helmets are cool and REALLY REALLY IMPORTANT IN COMBAT, I figured I would make it a free +1 AC. It still costs one inventory space.
Some weapons, like longswords, staves, and spiked gauntlets, add +1 to AC in you are skilled enough in their use.
Lastly, (thank you Goblin Punch for giving me the courage to do this) AC has a non-magical cap of 18. Why? Because I don't do number inflation. I wrote up a monster for my system the other day that is literally the size of a small planet, and it only has an attack bonus of +13. That doesn't sound high enough when you come from 3.5 for 4e or Pathfinder, but in my system, characters max out their attack bonus right around +6 or +7.
This post was kinda stream-of-consciousness. The next one will be a bit more put-together, promise.