Just out of interest and on the topic of realism, how hard is it to deliver a wound with a sword or other weapon against someone wearing full plate in real life. Do you absolutely have to find a gap in the plate or can you actually part the steel with conventional weapons?
With swords you have to find a gap in the armor as the force of the blow is distributed over a too large area to pierce armor.
See here (although that is non-lethal duelist fighting. Part 5 & 6 shows gap finding)
[ame=http://www.youtube.com/watch?v=1S_Q3CGqZmg&feature=related]Gladiatoria : Part 1/6 : Swordfight in Armour : Hammaborg - YouTube[/ame]
As you can see, fighting someone with plate armor with a sword does not look like any sword fighting technique we commonly associate with swords.
There were special weapons designed to defeat armor either by piercing them with a small hook like the Polaxe or other such weapons (picks, pikes, halberds, hook hammers, etc.) or by crushing the armor and possibly even the bone underneath it (maces). All of those weapons are vastly underrepresented in D&D.
About cost as balancing factor, why is that bad?
Its not as if earlier editions (3 and 4) did not use cost to balance magical items. And many other games use cost as part of the balance, too.
Back to the topic, I am also against unarmored, lightly armored guys having the same, or even comparable, AC than people in heavy armor. Light armor has some advantages (cheaper, can be worn for longer/in more situations, more silent, unrestricted movement,...) and thus should have penalties to AC (and not too small. Imo, wearing light or no armor should be a big risk when standing on the front line).