A suggestion for a system that would adhere to all of my guidelines coalesced in my mind this morning.
1. Each of the three categories of armor has a specific associated AC and Strength requirement.
Category AC Str
Light: +2 -
Medium: +4 13
Heavy: +6 15
2. Full Dex bonus in any armor.
3. Medium and heavy armor impose penalties to movement rate and all Dex checks (perhaps including Dex saves). An additional penalty of some sort wouldn't be uncalled for either.
4. Within a category are multiple types of armor (Medium might include chain, scale, ring, hide, breastplate, etc), all of which are roughly equal in effectiveness.
5. Individual types of armor vary in factors not listed above. Examples of individual variances might be:
a) +1 to AC verses one or two specific damage types (piercing, slashing, bludgeoning, fire, electricity, cold)
b) Reduced Dex penalty and weight
c) Reduced speed penalty
d) Perhaps 1 point of DR vs certain types of attacks
6. You can still have one "junk" armor (Padded, for instance) in the Light category. It simply doesn't get any special variant factors, but probably weighs little.
7. Some types of armor within each category are still more expensive, and heavier categories are also more expensive. Within a category, the armor types that are more expensive are the ones that weigh more and provide more direct (even though situational) AC bonuses. The cheaper ones are the lighter ones, which have a tendency to provide bonuses to speed and maneuverability.
Notes: Verisimilitude is maintained for NPC soldiers, since it's cheaper to outfit them with armors which are inferior in protecting low-level soldiers from getting hit.
Choice is maintained for PCs, since the basic features of each armor in a category are identical, and the variances allow PCs the option to specialize however they prefer, without feeling like they are locked into any particular type--not even by their Dex bonus.
