Now, I don't know if I'd go all that far-- my warrior has tanked plenty without being specced protection, so that's definitely possible. But he does have a point-- a shield is just plain necessary for tanking. If a tank doesn't know how to keep aggro, isn't ready to take damage or pull mobs off the healers, or thinks taunt is a damage spell, the group is in trouble.
Of course, most players aren't so nice to Xylox-- they say that though warriors do need to know what they're doing, so does everyone else. If a warrior can't keep aggro because the DPS is clueless, or doesn't get heals because the priest is in shadow when heals are necessary, it's not his fault. And still other players say that the reason lots of warriors don't know what to do right away is because tanking is one of the only skills in the game that you can't learn solo-- you've got to have a helpful group to teach you how to do it the first few times.
No one actually calls Xylox out for posting like he's the warrior master on his level 10 Shaman (that's what I'd do), but the thread really does provide some interesting thoughts from both sides. Bottom line: the best way to keep from wiping is to make sure everyone in the group knows what they're doing. All should be responsible for aggro control, all should know where to target DPS and when, and everyone should play their part well. It's true that an unskilled tank can be the first to mess up a group (because if she can't hold aggro, it's over right then and there), but everyone's got to do it right to get it done.