How I would tackle the armour question.
First, I must provide definitions.
Defense: I refer to the classic idea of a 'miss'. If an attacker fails to defeat my defense score their attack fails to land and does nothing / fails to break my magical defense and doesn't turn me into a toad / goes sailing past my character's head leaving him unscathed.
Mitigation: The damage that the attack would do to my hit points is lowered.
Absorption: The amount of damage that a character can absorb before they die.
Now the synopsis of the system.
1) attacks come in different types and different classes have a higher defense against those attack types: wizards are highly versed at turning away spells but not arrows, warriors are great against berserkers with axes but weak against devious sorcerers and their magic.
2) the bulk of a classes defensive score is determined by that class and level. The classic level 20 fighter, having just come out of a bath wrapped in a towel, is still very likely capable of getting out of the way of a feeble dagger thrust from a level 1 fighter (not reflected in their vast hp pool but instead in a high defense score). Throwing on a breast plate isnt going to make him get out of the way, it is going to:
3) mitigate! armour's primary job is to mitigate damage.
4) absorb! lastly, armour should provide a bonus to the amount of damage a character can absorb.
5) armour should also affect mobility, visibility and flexibility
That being said you can then create armor that has different properties that appeal to different classes.
Lets take the archer in light leather vs full plate.
Lets say that the archer, at a mid way level in the game, has a 10 point defense to ranged attacks and a 2 point defense against melee attacks. Basically in his type of warfare, shooting arrows back and forth, he excels. Get up in his grill and the archer is much easier to dispatch.
Plate armour could be designed to provide minor bonus vs melee, a serious movement penalty and moderate mitigation and absorption.
Leather armour could be designed to provide a minor bonus vs ranged, no penalty to movement and light mitigation and absorption.
In a situation where the archer can dictate the grounds of the fight (ranged) he is going to want to have maximum mobility and also maximize his ranged defensive attribute so that his foes could never touch him. Making leather armour the obvious choice. But, funny enough, if that very same archer knows he is going to have to go toe to toe with an enemy in a short hallway, he is going to choose plate.
As armour grows in power it is important to ensure that it never provides a substantial bonus to Defense. Leave that to the class type.
Give it instead, larger bonuses and penalties to movement, mitigation and absorption.
Design your light armour wearing classes to want to be able to move quickly around the battleground and maximize their ranged defense.
Design your heavy armour wearing classes to want to maximize mitigation and absorption at the cost of movement and defense.
Design your light armour system to make higher quality suits (say hide armour vs basilisk hide armour vs dragon hide armour) retain the core bonuses (ranged defense, mobility) while providing higher mitigation and absorption than the earlier iterations of the suit.
Design your classes to be able to take feats and skills that modify their defense.
At the end of the day, with this type of system, you could have a wizard type character that is naturally good at defending against magic purchase skills that makes them a bit more difficult to hit in melee and they wear plate. They kind of look like this:
Level 10 Wizard wearing plait: +10 defense vs magic (class bonus based on level), +5 defense vs melee (0 from class, 3 from elected skills, 2 from plate), +5 mitigation vs melee (again from the plate), +5 absorption (extra hit points from being encased in metal)
vs the robe type wizard, let's say with some sort of spellweave robe that protects against magic and having elected class skills that make then even better against magic.
Level 10 Wizard wearing spellweave robes: +15 defense vs magic (10 from level of the class, 3 from skills, 2 from spellweave robe), +5 mitigation vs magic (robe), +2 absorption (robes just dont provide as many hit points as plate)
Your plate wearing wizard would be pretty hard to kill with magic and still kind of hard to kill with a sword. This is reflected in their 10 point magic defense score and 5 point melee defense score.
Your spellweave robe wearing wizard would be near impossible to kill with magic but if you can right up in his face he can be taken out with a sharp stick. This is reflected in their 15 point magic defense score but zero vs melee.
Also important to note is the plate wearing mage has zero mitigation vs magic. So in those fireball situations where you can't get out of the way and just have to absorb, the spellweave robe wearer is going to do better vs magic.
And that's my ramble.