My husband used to tank on his Shammy until Dire Maul was released. But then, in those days, I was trying to mainheal as a resto druid when all I was MEANT to do was Innervate ;P. We were just gluttons for punishment, but it was possible.

As far as the whole +threat thing, I'm kind of surprised it's still in game, but I'm guessing it's meant more for "kite-tanking" not real sword+board tanking. More for the new instances than what we currently have, anyway. UW:Rockbiter, Frost shock, Ghostwolf, rinse and repeat.