One thing that we've been taught as tanks throughout most of World of Warcraft is that you had to be capped at something in order to not be insta-killed by bosses. Back when I started playing, this was referred to as reaching uncrushable. You would have to stack up 102.4 percent avoidance in order to push the dreaded crushing blows off of the boss's list of possible attacks he could hit you with. This also meant that all incoming hits were avoided or mitigated in some way, shape or form. That primary form was blocks, and abilities like Shield Block and Holy Shield at that time were custom tailored to this environment.
With the release of Wrath of the Lich King, crushing blows were deemed a thing of the past and bosses would no longer be employing them against tanks. However, we quickly had a new cap to deal with instead of that called the defense cap, aka reaching uncrittable. This meant accumulating 540 defense skill against raid bosses in order to remove bosses of being able to hit us with a critical melee attack. In early raiding tiers, this was a constant balance between gems, enchants and trinkets, as each piece of gear we accumulated could change how the scales tipped.