Addressing this at an endgame level won't work. I think lots of newbies who try tanks either don't know that their class is actually a tank, or even what a tank is. The game is terrible at explaining this stuff, and the optional Hall of the Novice doesn't do enough to help.

There's also the fact that tanking is really annoying and hard until you get your tank stance. This is wrong - holding aggro in Sastasha/Tam-tara/Copperbell shouldn't have to be a challenge at all, because it's the first time these newbie tanks are exposed to the enmity system in any real way. Playing a DPS job is fairly straight forward as an abstract concept - you just deal damage. Playing healer is very clear, and I think most newbies understand what a healer does. But I don't think the intricacies of tanking are properly explained during the early leveling process of these classes, which is a huge problem.

tl;dr - Tanking at a low level in FFXIV isn't explained as a concept or taught well enough by the game itself, and is too hard in early dungeons where there's no tanking stances. Of course, aggro management in early dungeons doesn't matter because nothing hurts, but it's still a fact that a tank that isn't holding aggro will get negative attention anyway.