What do you guys think is the cause of severe lack of Tank users?

[Shurrikhan]

Yeah, Shield Oath did, but not Defiance.

I could have sworn it had a minor one, too. It most definitely got one before the 2.1 rework, as you'd still start each trash pull in Defiance (yes, even without Unchained), grab up some heals and a Regen or shield while your 4/5 or 5/5 Wrath stacks were raising healing taken, Inner Beast to top off, and then drop Defiance for further damage during Bloodbath. Admittedly, my Warrior was already at 50 going into ARR, whereas I spent the MSQ leveling my PLD (starting at 35 or so, iirc), so the latter is fresher in my mind, but I'd done enough no-heal WP/AK runs as Warrior prior to the 2.1 changes (marginal dungeon nerfs for notable raid buffs) to remember that much.

That said, given how much larger Skull Sunder and Butcher Block's potencies AND enmity modifiers were, and that Overpower had a modifier that could at least scale, AND that it didn't have the further penalty of just not being in Sword Oath, it'd make sense if Defiance's modifier was noticeably lower than Shield Oath's.

[Vyrerus]

I could have sworn it had a minor one, too. It most definitely got one before the 2.1 rework, as you'd still start each trash pull in Defiance (yes, even without Unchained), grab up some heals and a Regen or shield while your 4/5 or 5/5 Wrath stacks were raising healing taken, Inner Beast to top off, and then drop Defiance for further damage during Bloodbath. Admittedly, my Warrior was already at 50 going into ARR, whereas I spent the MSQ leveling my PLD (starting at 35 or so, iirc), so the latter is fresher in my mind, but I'd done enough no-heal WP/AK runs as Warrior prior to the 2.1 changes (marginal dungeon nerfs for notable raid buffs) to remember that much.

That said, given how much larger Skull Sunder and Butcher Block's potencies AND enmity modifiers were, and that Overpower had a modifier that could at least scale, AND that it didn't have the further penalty of just not being in Sword Oath, it'd make sense if Defiance's modifier was noticeably lower than Shield Oath's.

Nah, it just didn't have one. WAR just had bigger hits and more damage going out than PLD on weaponskills. The Maim buff was +20%, Berserk +50%, and then you had the 20% crit rate increase from Internal Release. Crits got that 1.1x enmity modifier, and then you could spam Flash when you went Pacified. Original Defiance was -25% damage dealt for +25% HP each Wrath stack adding 5% healing received, and 2% crit rate, but I can't remember if that came later. I know it was changed to 2% parry in HW with the crit going to Deliverance.

I do also remember using Flash first for pick up a lot, because Overpower had that weird animation delay until they made it snappy after 2.1 sometime.

[Shurrikhan]

Original Defiance was -25% damage dealt for +25% HP each Wrath stack adding 5% healing received, and 2% crit rate, but I can't remember if that came later.

Crit and healing were both there from the start, I think? And it was up to +25% maximum HP and healing received, which the change just made flat at 4 stacks, despite leaving the penalty at 5 stacks.

I just, again, am like... 80% sure there was an enmity modifier there as well, if only attached to the Wrath stacks just as max HP and healing taken was (in the style of 1.x Rampage and Berserk)?

I do also remember using Flash first for pick up a lot, because Overpower had that weird animation delay until they made it snappy after 2.1 sometime.

It did indeed. There was a LOT more swish (and swirl) before its flick when it still followed the 1.x animation. Plus, what else were we going to use MP on, especially when we had no Bard for Army's Paeon?