Take a couple of rules:
- armour encumbrance does not affect your AC
- equipment encumbrance does not affect your AC
- encumbrance does affect the maximum amount of DEX bonus you can apply
- armour and equipment encumbrance affects your skill checks as per a check penalty
Now that doesn't seem quite right to me. A fully armoured fighter has less a chance of dodging a swinging axe than an unencumbered one. I've seen players equip everything AND the kitchen sink. It doesn't quite seem fitting that no AC penalty is applied. I can see the benefit in simplicity doing it like this brings (sort of), but we're already adding like three values and checking a DEX cap.
I understand armour encumbrance not affecting AC because it is probably already factored into the AC value. But not considering extra equipment encumbrance is just crazy. AC is a strange value. It is both the resistance to being contacted by a weapon and the protection from damage from said contact.
It is this overlap of concepts that got me thinking in breaking combat into to parts. Achieving actual contact with the target and determining the armour's effectiveness at stopping the damage. On one side there's the "hit roll" which does not include armour bonuses. Actually it includes armour penalties. All that which makes you easier to hit due to having armour. On the other side there's the "soak roll" which counters the "damage roll". Damage rolls are only done after the target is hit. The soak roll depends on the armour and is clearly better the better the armour is.
So heavier armour works against you in the hit roll, but for you in the soak roll. Lighter armour works for you in the hit roll, but against you in the soak roll. Thus, by separating this two rolls it is much easier to factor in elements like total encumbrance and armour quality. A light elven armour of great quality is both beneficial in the hit roll as it is light and of low encumbrance and it is also beneficial in the soak roll due to its toughness.
The point behind this is to provide a counterbalance to excessive armour and thus min-maxing. To provide an incentive so players take a moment to think their gear layout and armour. An incentive so players can consider unarmoured play a real possibility. Take the following example on a ship.
Travelling fully armoured on a ship is extremely dangerous. Fall overboard and by the time the ship comes around you're a permanent part of the coral reef. On the other hand in D&D fighting without armour is extremely risky. Chain mail gives twice as much benefit as leather and full plate 3 times more. Currently there is no clear incentive to use leather over chain mail in D&D. Chain mailed characters are not hindered enough to be worth the risk. If you have a +2 dexterity bonus it still adds up with chain mail. That's because for the average and slightly above average character (most of them) there is no encumbrance penalty for amour usage.
If you break it into "hit roll" and "soak roll" both characters have the same odds of getting hit (without applying any penalty for chain mail). While your soak roll will be less effective with leather than with chain mail you can still gamble on not getting hit in the first place, and thus still survive a sea boarding. In D&D you have twice as much chance of getting hit and thus suffering damage if you use leather. Clearly this promotes heavy armour usage.