I read the thread about tying AC to damage reduction, and while I agree with the intent of making higher levels of armor class more useful/viable, I find the idea of tying it to fortification problematic, for reasons other people have already discussed cogently.
I would like to suggest an alternative to changing around the entire fortification system.
Tying higher levels of AC to "bonus" Damage reduction.
For example base AC is 10. So at say AC 15, add 1 bonus DR (for physical damage), and 1 additional bonus DR for every 5 points of AC above that, such that 2 DR at AC20 up to say 8 bonus DR at AC 50.
While it might seem rather underpowered at first glance, this is bonus DR, so it would stack with other forms of DR. It gives classes who naturally stack AC into the mid ranges (like Clerics for example) a reason not to just go cloth. And it provides a bonus to Sword and board fighters and AC style fighters that would be useful without being completely game changing.
Of course, like any idea, the actual numbers would have to be tested and tweaked...