So what exactly IS the issue with Warrior?

[Giantbane]

*** PLD will receive 120% Healing in Shield Oath.

Not quite. PLD require 20% less healing due to Shield Oath because they take 20% less damage. But this isn't what you're saying. For a fair comparison, you need the inverse of this. How much more effective HP does the PLD have because he is taking 20% less damage, and how much more effective healing is the PLD receiving?

1.0 / 0.8 = 1.25

PLD has 125% total effective HP and receives 125% effective healing due to shield oath

That makes no sense.
If Warrior DPS is equal to PLD DPS in sword oath, and Warrior DPS suffers a 25% damage debuff compared to 20% Paladin debuff, they would still be doing the same damage.
The logic makes no sense.

WAR should be doing more in tank stance for 2 reasons.

1) the PLD suffers more than the WAR when switching stances, the PLD not only loses the 20% due to Shield Oath, he's also losing something like the 16%? (no idea, i think kitru had it around this during one of his/her math fests) from the loss of Sword Oath (<---- EDIT, clarified that the additional 16%? was from loss of Sword Oath).

2) Defiance isn't exactly a straight 25% penalty either. Defiance also brings 10% boosted crit with full wrath. This almost makes up for the 5% base difference between defiance and shield oath by itself. However, defiance also allows the use of inner beast (a solid dps move with no penalty when used with infuriate), and unchained (which can be stacked with berserk & Inner Release for some excellent burst).

So in ideal conditions the WAR should be pulling ahead while tanking.