The recent thought that wandered in is pretty simple:
The bonus to AC from the material armor and shields, not magical enhancements, is doubled against a critical confirmation roll.
The only drawback to this is that you have to allow the same for weapons made of the same materials. By this logic, an adamantite sword is going to have a big whopping bonus to confirming critical threat rolls against anyone not wearing adamantite armor! This premise just opens up a whole new can of worms without solving the main concern about heavy armor.
The idea of armor reducing critical threat range and damage is flawed. According to that idea, the armor is affecting the capabilities of the sword, which armor does not do. Armor protects the wearer from damage, it may reduce the effectiveness of a sword attack (for example), but it should in no way interfere with the function of the sword--which is what the proposal to reduce threat range/damage does.
Just going to play Devil's Advocate here a bit:
Bracers of armor +8 = 64,000 gp
+5 full plate = 26,650 gp
Guess which one gives you higher armor class. Hint: it isn't the more expensive one. Is it not ironic that I can buy +5 full plate and a +5 heavy shield for LESS than bracers of armor +8 and get more than TWICE the AC bonus? Doesn't help me against incorporeal attacks you say? No problem. I'll add on the ghost touch ability to my armor. Oh look, my AC against incorporeal touch attacks is now BETTER than it would have been if I had been wearing the bracers of armor +8, and I'm STILL saving money.
Don't forget to add in the extra 25,170 for your +5 heavy steel shield. For a total of 51,820gp. If you add on Ghost Touch (and you did say add, instead of change to +2 with ghost touch), this brings your armor to a whopping 65,650gp, plus your shield, 25,170gp, makes it 90,820gp. You could stop there, but if you had deep enough pockets, you'd want to ghost touch your shield as well, making it 64,170gp, and your overall total to 129,820gp!
Yes, you would have a +20 bonus to AC against corporeal and incorporeal attacks (+8 full plate, +2 shield, +5 bonus for each). But you would also be out enough gold to wreck the economy of a small or medium sized country as well as suffering a -6 armor check penalty (-12 to Swim checks), have a max Dexterity bonus of +1, a 40% arcane spell failure chance, movement reduced to 20' (if you're Medium), run reduced to X3 and 65# of weight carried around. He could have a +1 Dex bonus to AC, for a total AC of +21 against corporeal and incorporeal attacks, but otherwise an AC of +11 against touch attacks, +20 against flat footed.
Let's say your schmoe with the bracers is a wizard. For those bracers, he's either very lucky to find them or powerful enough to make them. We'll go with powerful. So, those 64,000gp bracers, now only cost 32,000gp since he can make them himself. Let's say, he's insecure still, so he makes an Amulet of Natural Armor +5, cost 25,000gp, for a total so far of 57,000gp and a +13 bonus to AC. That's still not enough, so he makes a Ring of Protection +5, 25,000gp for making it himself. So far, his bonus is now +18 to AC with a cost of 82,000gp. Now, this wizard just happens to be an elf (Dex 20/+5) who happens to also make Gloves of Dexterity +6 (now Dex 26/+8), for a paltry 18,000gp. This brings his total AC to +26, with only 100,000gp out of pocket. With some pocket change left over, the wizard creates a wondrous item, perhaps a cloak, with the Shield spell on it. This costs 1,000gp (caster lvl 1 X spell level 1 X 2,000gp X.5 for making it himself), gives a +4 shield bonus to AC, also good against incorporeal, and immunity to Magic Missiles. His AC is now +30, having spent 101,000gp. Assuming this wizard has a budget on par with the armored guy from above, he could shuck out another 18,000gp for a Periapt of Wisdom +6 which makes his above average Wisdom (14/+2) bumped up higher (20/+5). Having gained enough power as a spellcaster, our elven wizard settles to a more aesthetic lifestyle, becoming a Monk, where his new training grants him a bonus to his AC equal to his Wisdom modifier (+5). Our paranoid, elven wizard/monk has now spent 119,000gp (10,820gp less than the warrior above), has a final bonus to AC of +35, +30 against touch (and incorporeal) attacks, +27 if flat-footed, has at least a move of 30', run of X4 (X5 if he took the Run feat), protection from Magic Missiles, no arcane spell failure and no significant weight carried around. AC is incredibly better all around with no restrictions on mobility or spellcasting, and for over 10K less than what Mr. Armor above forked out. And if they had to walk the plank, one would sink, the other would swim (or fly away, or levitate, or teleport, or polymorph, etc., etc.). I've left out Dodge and other feats and things that could be acquired equally by either side to mundanely boost AC even further.
Both examples are in the extremes, but the point was to show that while armor can rock, it doesn't out-rock.