Warrior: Shield Barrier. On live, I have a 1.3 MILLION absorb with 60 rage and 500k Vengeance.
Paladin: In ToT, Sacred Shield was hitting absolutely insane numbers when solo tanking Tortos and Ji-kun. It may not be something they use for every fight anymore, but it's still an on demand absorb.
Monk: Guard. I don't even need to say anything else.
DK: With 105% Mastery the bubble from Death Strike is anything but small.
My statement stands. Every other tank spec has an on demand shield. I never said that shield was tied to their AM (It is for 3/4 though).