There WILL be a TL;DR version at the bottom, of course.
As many people are no doubt aware, Patch 4.2 will bring with it a change to Holy Shield for Protection Paladins.
Holy Shield has been redesigned. This talent is now an activated ability off the global cooldown. It grants 20% increased block amount to a paladin's shield blocks for 10 seconds, with a 30-second cooldown.
Simple math indicates that this is an overall nerf.
Instead of 40% damage reduction with a 100% uptime, you have 50% damage reduction with a maximum 33.33% uptime.
Look at it like this. Imagine you have a 100% block chance, and are facing an enemy that will deal 1000 damage every second to you.
Under current conventions, the damage stream will be flat - 600 damage per hit.
With the change, however, this alters the flow. For 10 seconds every 30 seconds, the damage will be 500 damage per hit. At all other times, it will be 700 damage per hit.
Over short-term, this is a net reduction. However, at the 20 second mark, it is equivalent. Beyond that, it decreases, and at the 30 second mark - where the cooldown is available again - it is markedly worse.
Over 30 seconds, a 4.1 Prot Paladin would receive 18000 damage. Over 30 seconds, a 4.2 Prot Paladin would receive 19000 damage.
Okay, so it's a nerf. And nerfs happen all the time. It's a fact of life in this game.
Well, the question then becomes "why?"
Paladins and Warriors are two of the three shield-using classes in the game. The other class, Shaman, isn't a tank-capable class (or at least, isn't designed with that goal in mind), and so the natural inclination is to draw a parallel between Warriors and Paladins.
Death Knights and Druids use other abilities to simulate the damage reduction granted by shields, in order to balance the classes. These abilities are difficult to draw parallels to, and so we'll take Death Knights and Druids off the table, and compare Warriors to Paladins.
Protection Warriors and Protection Paladins both use mastery to increase their shield block chance.
For Protection Paladins, upon learning Mastery, you are granted 18% shield block chance, and an additional 2.25% for each point of mastery you have. (More accurately, you are given 8 mastery to start with, which accounts for the 18% shield block chance, as 2.25x8 = 18)
When Protection Warriors learn Mastery, they are granted 12% shield block chance, 12% critical block chance, and 1.5% chance of each for each point of mastery you have. (Again, they start with 8 mastery, 8 mastery = 12% chance.)
Each shield-using class (including Shamans) is also given a base 5% chance to block.
In addition to this, Protection Warriors have a 'baked in' 15% additional chance to block, from from the 'Sentinel' passive ability.
The long and short of all this, is that at 20 Mastery (base 8 + 12 from gear), both classes will have a 50% chance to block attacks. I believe this is the point to which scaling was intended.
Prior to reaching this, Warriors will have a greater chance to block than a Paladin in identical gear, and afterwards, a Paladin will have a greater chance to block than a Warrior in identical gear.
Since the 50% mark seems to be the 'magic number', we'll work with that. It's at that point where gear benefits both classes to an equal amount. Prior to/subsequent to that point, scaling works a little differently.
When comparing the two classes, it is important to identify and illustrate the differences in the respective mastery effects.
Without the use of cooldowns (or gear, such as the 1% block value meta gem), a Protection Paladin in 4.1 - provided they are keeping Holy Shield up, which is done by using Shield of the Righteous or Inquisition (or, with appropriate talents, Word of Glory) - will block for 40% of incoming physical damage upon a successful block.
This is a static value, and will only vary if the Paladin's rotation changes, at which point, it becomes 30%. For the purposes of this experiment, let us assume that the Protection Paladin's rotation is constant, and the uptime is 100%.
In other words, assuming no other avoidance stats (which doesn't happen in a real setting, but for the purposes of simulation, we will use this), there's a 50% chance that an incoming physical attack will hit for 100% damage, and a 50% chance that an incoming physical attack will hit for 60% damage.