D&D Armor As Damage Resistance
Douglas Cole wrote about this on Thursday - essentially, porting the approach of GURPS combat to D&D 5e.
Here is how I think it could be implemented.
Armor reduces damage, bonuses improve AC - Armor doesn't provide improved AC. Only Dexterity, magical protection, shields, etc. provides improved AC. Actual armor (including pluses from magic armor) just reduces damage via a mechanic called Damage Resistance (DR).
For example, Plate is AC 18, no DEX bonuses. You get +2 from DEX, +1 from a shield, and +1 from a Ring of Protection. Because of the heavy armor, you ignore the DEX bonus. You have AC 10+1+1 = 12 and people roll vs. that to hit you. If they do, subtract (18 - 10) = 8 from their damage.
Chinks/Gaps in Armor - You can attempt to bypass some of the armor of your opponent. The attack is at Disadvantage, and you can't do this if you are already attacking at Disadvantage. If you hit, your opponent's DR is halved, round up.
There are a few challenges.
Monster AC - You'll need to note the DR of monsters, which would be AC - Dex adjustments - non-armor bonuses - 10. So a critter with AC 16 (Natural Armor) and a DEX bonus of +2 would have 16 - 2 - 10 = 4 DR.
In addition, you need to know if their armor is Heavy, and thus is already forgoing a DEX bonus, or not.
Balance - D&D and its hit points are balanced around the abstraction that armor reduces hits, not reduces the damage of hits. Low damage monsters will become harmless, which reduces the effects of bounded accuracy (the relatively low ACs of 5e monsters.)
Lots of Decision Points - You will need to constantly evaluate magic items, effects of powers and spells, and so on for their ability to reduce hits (increase AC) or reduce damage (increase DR) - nothing can be used just straight-up.
These aren't small, but springing off of Doug's idea, this seems like something ready for playtest and to run with if you prefer the GURPS-like effect of armor reducing damage instead of reducing the number of hits.