In Battle for Azeroth, there are some major changes being made to tanks' primary active mitigation abilities. Four out of the six tank specs are going to have AM that works in the exact same way (with some specs having additional effects, but the primary effect is the same). Active mitigation for Guardian Druids, Protection Paladins, Vengeance Demon Hunters, and Blood Death Knights all increase armor by some multiplier of their primary stat. For example, Ironfur (Guardian): "Increases armor by 35% of your Agility for 7 sec".
These abilities only reduce physical damage, and in particular, damage that is reduced by armor (e.g. the vast majority of bleed damage is not reduced by armor despite being physical damage). Since armor is a percent damage reduction, these do scale with the amount of damage you're taking. Also, since these abilities scale with primary stat (Strength/Agility), your AM benefits from things like trinkets with primary stat procs, but not from upgrades to rings for example (since they do not have primary stats). Since the vast majority of gear does have your primary stat, your AM more or less scales with item level.
This got me thinking; Warriors have AM that does not scale with primary stat. It does scale with damage taken as Shield Block is a % damage reduction, but better gear doesn't make the ability stronger. I wanted to see how Shield Block compares to these other forms of AM, so I made this spreadsheet with some graphs to show the amount of damage prevented under various conditions. The first two graphs include damage reduction from armor, while the last two do not. Note: In the graphs where primary stat increases (the first two), armor is increasing as well. This is simulate item level increasing.
Looking at this first graph, it appears that as you gain more primary stat, active mitigation gets stronger. However that is actually not the case, as we'll see in the third graph. The curves of these lines come purely from the damage reduction provided by armor. Stating the obvious, as you gain more armor, the damage that is prevented increases.
In this chart, armor and stats are constant, and we're still combining the effects of AM and armor. Unsurprisingly, as damage taken increases, the damage prevented also increases since armor is a % DR.
This is where things get pretty interesting. For this graph, the effects of the AMs have been isolated, so we can actually see what each is doing. Shield Block of course has a slope of zero, as it does not scale with primary stat. But, the other abilities also have a slope that is very close to zero. That means the defensive value stays the same despite your primary stat increasing. At first this doesn't seem to make sense, but remember that in these graphs armor is increasing along with the primary stat. So essentially what is happening is that the scaling of these abilities is counteracting the diminishing returns on armor. All of these abilities are % DR; they don't remove a flat amount. In other words, these abilities remove the same portion of damage, regardless of item level. Now, if primary stat was the only thing that was being increased, you would see a positive slope on the primary scaling AMs.
As you've probably noticed, Shield Block is about 5 times stronger than most of the other AMs. However, if you look at the first graph, you'll see that when combined with the effects of armor, the total DR is much closer.
So what is the point of all of this? Well, the main takeaway is that Shield Block doesn't outscale or get outscaled by any of the other abilities, despite not scaling with primary stat. They all actually scale in a very similar fashion. And most interestingly, in my opinion, the % DR provided by these abilities stays the same despite item level increasing.
It's very important to realize that looking at these abilities in a vacuum like this tells you absolutely nothing about tank balance, or even the balance between the abilities themselves. Overall uptime, the additional effects, etc, all play a huge role in actual balance.
Finally, some Warrior specific feedback. While not scaling with primary stat is okay with regards to the actual scaling of the strength of the ability, it is frustrating that we get no benefit from primary stat increases when the other specs do. I see no reason not to change Shield Block to scale with primary stat in some way. Personally, I would be fine if Shield Block increased armor by some percent of strength, just like the other abilities.