I'd like to talk about the accessory problem in regards to all roles, not just tanks.
The design choice to only include 1 main stat on them has worked for now but as we creep into ever higher item levels that comes with continued content and progress, there are issues that are going to come up unless they are changed.
Right now for Tanks, 5 slots of gear are devoid of any sort of attack power increase. With 320 accessories, this represents 115 points times 5, or 575 points in total. The issue does not lie with this exactly, but with the fact that when i330 comes out it will likely increase to at least 122 (310-320 saw a 7 point bump.) This increases the difference to 610.
The way the accessories are done in the game currently, attack power disparity will continue to increase, and the gap will continually widen. 575 now, 610 later, and if gear gets more stats faster, so too will the gap accelerate.
With threat being intrinsically tied to damage, with no changes we will eventually see a point where staying in tank stance, pressing threat combo only, and using shirk / shadewalk shenanigans will not allow tanks to hold aggro as the DPS attack power will be that much higher.
With no changes to accessories, it necessitates continuously increasing enmity generation through patches and hotfixes to keep up.
The same goes with HP.
Currently DPS and healers are 600 points of vitality behind tanks, with the gap increasing with every tier of content.
Eventually we will end up with tanks that have 2x or more raw HP than the rest of the party. This doesn't sound bad on paper, but it's a nightmare when you start to think of the implications this disparity introduces.
Raid-wide damage will either be tuned to actually make a tank care (and smashing the rest of the raid dangerously low), or they will have to accept that the tank simply will not give a rats bum about anything other than auto attack damage and busters. Alternatively we will see an extreme increase in the # of % hp based raidwide damage, which negates a party's feeling of progression.
Even as a DPS, it feels nice to have done a fight at lower gear level and be panicked about the hp you drop to after certain mechanics, only to do the fight with better gear later and survive comfortably. It's not a good feeling exclusive to tanks.
Additionally, world content becomes an issue, and the question of "Do we make this require a tank?" becomes larger and large. If tanks have such extreme hp relative to the other roles, then damage tuned to hurt them will by extension exclude anything but a tank actually taking a hit from the thing. Hunt mobs and fate bosses would completely eradicate even DPS's with healers stuck on them.
The question of "Do we make this require a DPS?" becomes relevent too, as they would need to decide if they should make a monster's HP actually decent (and making it take extreme amounts of time for a tank to kill) or to make it reasonable for a tank to kill and be a complete pushover for dps.
Neither of these scenarios is actually good, and makes balancing the game in the future a nightmare.
I'd propose that all STR, DEX, INT, and MND accessories provide 25-50% of the vitality bonus present on VIT accessories, and to allow Vitality accessories to provide 25-50% of the STR bonus present on Maiming/Striking accessories, and to lock out melding any main stats on them.
This would keep HP values on the other roles on the same scaling rate as a tank, while still putting them at a fixed x% behind tank HP.
This would also keep attack power values on the same scaling rate, while keeping tanks at a fixed x% behind DPS.
The amount of Vitality present on left side pieces and weapons could be adjusted if they wish to change the HP gap, and the amount of STR present on left side pieces can also be adjusted to change the attack power gap on a per tier basis, allowing SE to have much greater fine-tuned control of where they want HP and DPS balanced for any given tier of content.
This future proofs against the need to gimmick out % based damage, patch in constant enmity increases, and eliminates the headache of trying to balance world content.
TL;DR: Having HP and Attack Power on different scaling values will eventually lead to insane balancing headaches. Keeping them on the same scaling but a fixed percentage ahead/behind of the other roles eliminates all of that.