Tanking bosses with heavy magic damage (especially Nef)
I noticed a rather interesting gem the other day, the Effulgent Shadowspirit Diamond.
(81 stamina and 2% less magic damage taken) No one on EJ seems to suggest it and for tanking something like BH, it's not gonna be great, but for something like Nefarian, it might be quite a bit better. According to some logs of Neffy kills, tanks take about 40-50% magic damage. So 2% off magic damage might be better than an extra 2% armor (was about 1% physical damage reduction for me) or the 1% block value. (Not entirely sure how to calculate warrior block value into physical damage reduction, so I'm unsure about that one)
Also, we seem to use cauldron flasks for everything, which are ideal for most classes, but not really tanks. 450 stamina will give you roughly 5% extra health (at 8k stamina)
The alternative is to use an Elixir of Mastery (225 mastery, gave me 3.32% block as a pally, 2.22% block+crit block as a warrior)
and either an Elixir of Deep Earth (900 armor, 0.62% phys. damage reduction on pally and 0.74% on warrior)
or a Prismatic Elixir (+90 magic resist, which gives 15.6% magic damage reduction at 0 resist, 10.75% on top of kings/motw resists, or 8.21% on pally/shammy resist auras)
Note: your numbers may be different due to total values of armor/mastery and diminishing returns.
Also the Prismatic Elixir will give a lot less mitigation if you've got the Mirror of Broken Images use effect on.