To give you an idea of just how good defensive disc is, my sperglord calculator suggests that to tank Derakor the Vindicator:
- BIS Warrior with Defensive: 6.5s chain
- HOT geared Warrior with Defensive: 5s CH chain
- BIS Paladin: 3s CH chain
- HOT geared Warrior without defensive: 2.5s CH chain
- HOT geared Paladin: 2s CH chain
So you can see that defensive warriors simply crush Paladins for pure beefiness, and Paladins' superior snap aggro is completely pointless when Warriors can have 40 aggro spell charges stored on Puppet Strings, Midnight Mallets, Bladestoppers, and Prayers of Life and recharge them for 100p a click or less.
I think raid geared paladins can tank slowable NTOV dragons, but in practice no one does this and as a Paladin you'll probably face an uphill battle just to tank wurms in the Halls of Testing. Also, contrary to Swish there is no such thing as offtanking unless you are talking Hate or Arena trash that a Shaman or Ranger can handle easily.