@Grak

It's important to note that magic damage doesn't throw effective health out the window and make it strictly about stamina. Magic damage just changes the still-complex calculation used to determine effective health. A tank might have 50,000 health, but that's not his effective health even against magic attacks. Take my paladin for example, and assuming I actually had that much health to start with....

TALENT: Improved Righteous Fury: all damage taken is reduced by 6% . . . Now my effective health is 53191

BUFF: Blessing of Sanctuary: Reduces damage taken from all sources by 3% . . . Now my effective health is 54837

TALENT: Argent defender: all damage taken below 35% health is reduced by 20% (works out to a flat 8.75% increase in effective health) . . . Now my effective health is 59635

TALENT: Shield of the Templar: Reduces all damage taken by 3% . . . Now my effective health is 61479

I won't even go into the effects of divine protection, because that's just begging for an argument about what should and shouldn't count towards effective health

Other classes have different sets of talents and buffs unique to themselves. I've seen DK tanks that have two separate tanking specs, just because unholy has more magic mitigation so they switch to it for that kind of boss, then back for physical bosses.