Defining Block Value as Mitigation
Shield Block Value and additional Effective Health
The Effective Health shown on this website is reliable and accurate. However, there is an additional, scaling bonus to your Effective Health in the form of Shield Block Value. This is the secondary scaling of Shield Block Value (the primary scaling is already included in the equation here).
In addition to increasing your Effective Health by a base amount -- exactly like Health -- it also decreases a percentage of damage taken. However, the percentage decrease cannot be quantified without detailed information regarding the attack speed, average damage, and maximum damage of your enemy.
Consider the difference between going into a 5-man or Heroic instance with 500 Block Value and going into a fully realized raid boss encounter with same. The percentage reduction -- and your total Effective Health -- will be different for each encounter. Where you might gain limited advantages to Mitigation against some bosses, others, such as the Prince in Karazhan, will yield far better returns with Block Value. In some cases, especially in world farming, Effective Health can multiply several times over due to a state of near-invulnerability.
Shield Block Value is the stat with the highest potential in the Effective Health equation. It shares a unique relationship with both Health and Armor. Armor is a curved mechanism for increasing Mitigation; Shield Block Value, however, is a linear mechanism for increasing Mitigation. In other words, Shield Block Value does to Mitigation what Health does to Life.
Shield Block Value gains the same benefit as Health when factoring in Armor. As Armor increases, the value of each point of Shield Block Value increases. Then, as the value of Shield Block Value increases from Armor, the value of Health and the impact on Effective Health increases unpredictably -- unless we know the incoming damage values.
However, keep in mind how limited Shield Block Value is on gear. Reaching 1,000 Shield Block Value, for instance, is virtually impossible without the use of trinkets or the Tier 5 Shield Block Value bonus. Blizzard most likely itemizes large amounts of Block Rating -- a near worthless stat for Warrior raid tanks -- to keep item levels and potential Block Value in check. Unfortunately, this also extends to gem choices, and it is this reason -- not potential Shield Slam damage -- that we are unlikely to see many Block Value gems on the horizon.
Complex? Sure. Bottom line: don't write Shield Block Value off for it's Mitigation value.
Thus, Avoidance also helps reliably keep Effective Health high. By increasing Avoidance such as Dodge, you simultaneously decrease the chance of incoming attacks not being avoided or blocked. Indeed, increasing Avoidance will keep more Shield Block charges active for longer, and with enough of it, you become virtually immune to Crushing Blows in most encounters.
This isn't to say that you need to go gem everything with Avoidance. Please note the wording in bold -- it will not increase Effective Health. It will only help stabilize your rate of effective Shield Blocks, which in turn allows Block Value to be reliably factored in. Heavy Avoidance is only important for some boss fights -- boss fights you would probably use some Avoidance gear for in the first place. Also, Warrior equipment is already unusually weighted with Avoidance stats, and you gain plenty through normal gearing.