Very good post, btw.
Hmm, even though the game can use sophisticated math to calculate hit and damage probabilities and potentials, we should take into consideration the players perception on a system. They need to be able to differentiate the different type of armor and evaluate them without needing to resort to math. Flat percentages are pretty good, relative percentages bad, I agree there. Though I prefer a bit more basic approach. Damage Reduction counted in points is more intuitive than percentages, IMO.
Generally the lower the range of the numbers you need to compare, the better. That's the only reason I liked THAC0 - it used numbers that usually ranged from 10 to -10, giving the player a clear average-ish good (0), and pretty big steps. D&D 3rd, like many know, gave players stats potentially into the hundreds - god awful.
A variant and maybe an improved system of piercing / slashing / crushing would do the trick?
You could have a graded system of DR like so:
Piercing §-------------X------------§ Crushing
..where the X marks where on the scale a weapon lies.
Then you could have armor that gives DR in two numbers:
Damage Reduction: 4 / 9
(4 against pure Piercing, 9 against pure Crushing)
So, a weapon like this:
§ ---------X------------------- §
..that lies in the middle leaning towards piercing (say, a shortsword), would yield a DR of about 6 against the armor.
Combine this with a personal favorite of mine: Exploding / Penetrating dice for damage, and we're on a roll.
Using that kind of damage ranges, you would see some occasional hits that penetrate the DR even from weak weapons, without it becoming a crit-only event.
Make the armor count towards a general encumbrance capacity (EV, calculated not only from weight, but also size, shape and general portability, and in the case of armor, freedom of movement), and make that affect all the fore-mentioned, and I think that would be quite agreeable.
Many of the minor inconveniences, like penalties to pick-pocketing or spell casting while wearing oven mittens could be included in a list and general assessment when clicking on the EV value in the corner of the inventory screen - not included into the item description.