This is about how Oblivion uses the DR system where due to skill/weapon/etc you always did a percentage of the damage. But DT was used in FONV which was helpful in making power armor feel like power armor. DT uses a minimum damage that a player must give to penetrate the the armor and do damage to the enemy.
The DT system can make it interesting to fight harder leveled enemies and make it more challenging. Also itwill help with the damage sponge problem we get at higher levels.
Example of how DT works:
1. Enemy has a DT of 5 armor. Your character does 8 damage with your 8 damaging weapon. The enemy recieves around 3 damage.
2. Enemy takes a swing at your DT armor of 2 and he using a weapon rating around 6 damage, you recieve around 4 damage.
3. Now if you were to use a weaker weapon of damge of 3 it would do no damage or very little of on the enemy.
4. If the armor rating and weapon rating are the same its usually the same outcome as #3.
They seem to have redesigned combat by alot, so maybe this is an aspect they have changed well.
I know I was truly afraid of Deathclaws when Obsidian implemented this system in FONV.
PS. I'm sorry if I left a bad taste in your mouth for suggesting another FO idea into Skyrim when so many others have. Meh.