I've had no problems tanking any content skill or gear wise until I did Garuda Ex a short while ago with a WHM. Now I will throw down some basic PLD knowledge to start so people know:
TL;DR, skip to the line with >>>>> if you don't care about PLD mechanics.
While shield oath does increase enmity by 30%, it also reduces dmg done by 20%. Only a few PLD abilities have what is called an enmity modifier. That means some abilities do more enmity x dmg than others.
Most PLDs will pull with shield lob, then fast blade > savage blade > rage of halone. Some will add in off the GCD abilities like circle of win and spirits within. I have mine macro'd to my rotation with a /wait line so it goes off between the GCDs. If I need to not have them macro'd (waking up sleeping mobs with circle of win, silencing with spirits within on T2 for trash, etc) then I have the base abilities I can swap on my bars.
So the enmity modifiers, right? Shield Lob, Spirits Within, and Fast Blade do not have modifiers. So with shield oath up, they do dmg x 130%. Abilities with modifiers (they say additional effect: increased enmity") will do more threat. Those are Savage Blade (x6), and Rage of Halone (x10). There is more theory crafted math somewhere with more numbers but this isn't the point.
>>>>> The first ability we use to generate threat and start our combo, even with off the GCD abilities, does not generate enough threat to keep aggro off of you if you have Regen + Medica II HoT + Spamming Cure II or Medica AS I'M PULLING. Keyword is right at the pull. Not 2 seconds after or before. A crit heal will seal the deal with you dying unless I provoke but that has a 40 second cooldown. Oh, and since a lot of people don't seem to understand provoke, I will explain that as well:
It puts us at +1 enmity over whoever had the highest enmity when it hit. So we also have to hit the mob right after we do it. Know what that means? It means it's like starting the pull over again but with everyone already engaged with dots, hots, healing, and attacks. If the mob pulled over to you and is out of our melee range, we have to shield lob and try to get in range. Provoking does not immediately make you safe. If we provoke and you get a crit heal, well damnit, have fun.
There is no reason for this madness. Regen maybe to help ease the initial dmg but spamming cures as I'm pulling when no one has taken dmg is a no-no. I have not established threat with my measly shield lob > fast blade > spirits within. This is not a skill issue. This is low enmity generation at the start that ramps up.
Once I've gotten in at least a savage blade and going into rage of halone, threat is solid. A fully combo'd rage of halone produces ridiculous threat.
The whole point of this is to better educate WHMs since they seem unaware that they also produce a metric ton of enmity.