[NvPinder]

10 armor is exactly equal to 1 resist all. The increase for every point of resist all/armor is also a linear increase in terms of % of damage taken reduced, there is no diminishing returns on this for every point put into it which is a bit unintuitive from the numbers sometimes (eg, lets say going from 0 to 100 armor gives you from 0% to 50% reduced dmg from all attacks, whilst going from 900 to 1000 armor gives you from 98% to 99% reduced damage from attacks, altho the last 100 armor only adds 1% absorbtion compared to the first armor adding 50%, in reality damage taken in both cases is halved by 50% total. (and remember, 10 armor = 1 resist all, there is absolutely no difference between them)

(Spoiler for those still struggling a bit, stolen straight from reddit, maybe he can explain better than i can)

+ Show +

charles is attacked for 100 damage!
charles has zero damage reduction!
---charles takes 100 damage!---
tod is attacked for 100 damage!
tod has 50% damage reduction!
---tod takes 50 damage!---
tod took half as much damage as charles!
tod can take twice as many hits as charles!
charles is attacked for 100 damage!
charles has 98% damage reduction!
---charles takes 2 damage!---
tod is attacked for 100 damage!
tod has 99% damage reduction!
---tod takes 1 damage!---
tod took half as much damage as charles!
tod can take twice as many hits as charles!
In terms of Diablo III, it takes 3,000 armor to get from 0% to 50% damage reduction. If you had 98% damage reduction from armor, it would take 3,000 more to get to 99%.
If you got 3,000 more after that, you would go to 99.5% damage reduction.

However, vitality DOES GET LESS EFFECTIVE for every point already added to it. adding 4k hp to your base 4k hp at level 60 literally doubles your suvivability straight up. Adding 4k hp when you're at 40k hp already tho however only adds an extra 1/10th to your survivability.

Since not many people realize the above, it leads to vitality being vastly overrated by a lot of people (including myself for a very long time) compared to the two other stats because of the noticeable change in HP being easy to account for ("100vit = 3.5k more hp??? WHOA! AWESOME!" kinda of thing, as opposed to "an extra 33 resist all is equal to some misc small amount of a % of reduction, whatever"). It's relatively important to strike a decent balance between both, because obviously rocking no vit with 5k hp but a ridiculous amount of resist isn't going to be as good as doubling your HP with 150 or so vitality for the cost of a tiny bit of resist all/armor, and similar goes for having way too much hp and not enough res-all/armor. This is why some people have >40k hp and struggle to survive act 2 inferno whilst others can have even much less but still breeze it.

I'm not 100% sure about other classes, but for wizards, all your armor gets a huge boost from energy armor which is basically a must-have in inferno mode for us, and if you get your gear well enough equipped, you can stop using force armor rune and instead use prismatic, which increases the bonus from resistances as well, and im sure other classes have armor/resistance boosting abilities/passives that they can take advantage of too.