My initial thoughts:
(Hear me out) At this point, most of the people I see that are oppose to tank stance are those who do savage and extremes. This is a very small population of the game as a whole, however the vocal majority of the forums. All too often, the dps-centric meta is thrown in our faces. You have dps with high hp, dps, and dps with healing capabilities. There is nothing else. Due to this, there has become a sense of blurred lines between the roles. Tanks focus more on damage output than mitigation. Healers prioritize their globals on damage rather than healing... there aren't definitive roles. I believe that in order to truly fix the tank stance debate, this should be addressed.
My proposed solution:
Give tanks a reason to use tank stance -- even if it's primarily used for the main tank: make auto-attacks hit harder, make tankbusters threatening, give tanks a reason to be tanks. As it stands, they just keep aggro and let their health pools do all the work while using the occasional defensive CD.
Give tanks a personal responsibility on encounter dynamics. It was mentioned earlier that many of the bosses reposition themselves throughout the fight. They're too scripted.
Give healers a reason to heal. I understand that one of the alluring aspects of healing for top-tier players is that they're not healbots. I want to stress that I'm not vying for that design. That said, most of the healing done is tied to oGCD abilities such as Lustrate, Indom, Earthly Star, Tetra, etc. That's not to say that GCD heals are non-existent, rather, they're less common. As with much of this post, I will share an unpopular opinion: I think heals should be less strong. Currently, healers can top a tank off with little-to-no effort. Cure II can hit for 35k+ on a 76K tank. That's almost 1/2 the health bar... Regen can crit for over 10k. Earthly Star can hit for 20K+ on everyone, Cure III can lolbenediction the entire party under the right circumstances... because of this, healers have so much downtime that all they can do is dps: maintain one or two dots and spam Stone IV, Malefic III, or Broil II.
My theory behind all of this is that it's a consequence of the 2.5 sec GCD. Because we are gated behind how often we can act, incoming damage has to be more scripted... it has to be manageable within a 5 second window (the time it takes to use an ability, then be able to reuse another ability). This has had a huge impact on macro gameplay (macro meaning large-scale in this case). Heals are absurdly strong, damage is less intense. Tanks can disregard their tank stances because they know that the next Essential Dignity will heal them for 40K. If they insist on keeping tank stances, they should make it such that they're actually needed... otherwise, they might as well rename them Tether... they're just as useful.
I mean no disrespect to any of the fellow forum posters. We may share a difference of opinion, but I want it to be known that my opinion is not any more or less important than yours. I value this discussion and all ideas and thoughts associated with it. Regardless of the path Square Enix takes, we're all going to reap the consequences. I hope, for all our sakes, it's for the better.