All druids get asked to heal, even bears who join the party with 10k+ health and 5k mana in caster.
PuG rogue: "who's the tank?"
Warrior w/ 4 digit health: "i am"
Feral druid w/ 10.5k health in caster: "Then who's healing?"
Retribution paladin/party leader: "can you heal?"
Druid: "Lemme go get my resto gear >.<"
Panzerkins have to rely on really good PvP gear to have the armor, stam, and crit immunity needed to tank while still having enough spell damage to build threat. I think they use Wrath-spam with 3/3 Celestial Focus and either Earth Shield or paly Concentration Aura to get 100% immunity to spell pushback from melee damage. Some of the problems with Panzerkin tanking include:
no positive threat mult (bears have 45% bonus, plus the ability mult)
low avoidance
no stam mult
no "taunt" (situational hinderance, most raid bosses are immune to taunt)
vulnerable to pushback from melee damage if the panzerkin loses Conc aura or ES.
vulnerable to silence