In the past, it has always been possible for classes like Warriors, Paladins and Death Knights to tank a normal dungeon, even without having a typical tank-specc. This holds especially true while leveling as few people chose to level with a tank-specc.
All you needed was a little defence gear to achieve crit-immunity. With the new Cataclysm design Crit-immunity is now backed into some talents somewhere in the tanking-trees, making it impossible for off-speccs to reach.
Now I'm not sure how big of a risk crit-immunity is for a tank but I think it's a bit of a shame. If the crit-immunity was backed into Defensive stance/Blood stance/Something Prot then it would go a long way for leveling groups.