stamina & resistance is the only counter to magic damage sources.
block is irrelevant against mobs that do magic damage.
physical damage mitigation, armor has more %reduction on the melee hit then a flat block amount. comparing stamina to block is pretty dumb since stamina is always applied 100% during a fight and can be modified more with buffs/consumes/cooldowns... block amount cannot.
block % should be thrown in with your armor reduction equation and defense to prevent crush/high damage melee hits.
block % is great against fast hitting mobs, shredding through shield block charges. (mobs with thrash ability and such)
drakes/nef do huge magic damage from shadow flame.. stamina is the best counter to this "after" you finish your melee calculation of defense against his melee attack/warrior call.
the 8k damage shadowflame during warrior call is always a fun thing to see.
for broodlord, armor is the best counter to his mortal strike.. block chance and defense help a ton, but if you're in might gear or less. when your defense fails you and mortal strike lands. armor and hp are the only thing left.
if armor is low (no inspiration/stone shield pot) expect 1 shot unless you used a HP % modifier before it like lifegiving/last stand.
during 1.6 release, our MT never got 1 shot by broodlord because we stacked armor heavily. we got his MS down to 5400 and less when the stars aligned with every armor buff on the tank and damage debuff on the boss. (demo shout and such).
can't wait to see the theorycrafting on vek'nilash with his -100 defense debuff every 10 seconds.