Well barring the exotic armors it seems like Light, Medium, and Heavy armors are designed around AC parity for High, Medium, and Low Dex characters.
Someone with an 18 Dex can get to 18 with mundane light armor
Someone with a 14 Dex can get to 16 with mundane medium armor
Someone with 10 Dex can get to 17 with mundane heavy armor, but suffers speed penalties.
So the High Dex character sticks to Light Armor because he doesn't get better with Medium Armor.
The Medium Dex character uses Medium Armor with a 1AC sacrifice if he doesn't want the movement penalty (or can't use Heavy Armor proficiently).
The Low Dex character uses the Heaviest Armor he or she is proficient with.
Light Armor, High Dex, Heavy Shield looks a little silly, but I don't think many dex-based classes are going to be trained in using shields.
Other than Dragonscale everything looks to be OK in terms of scaling AC.
It seems like heavier armor and shields ought to give you Disadvantage for things like sneaking, swimming, or climbing.
- Marty Lund