As a healer in this game, and long time healer and ranged dps in many MMOs it is your third point that keeps me from tanking. Also, I feel that healing is more like ranged DPS, so the transition to tanking is difficult. IMO Melee dps has more in common with tanking than healing or ranged dps.
I disagree with you on the issues of cost and blame, but I understand and appreciate where you're coming from:
Cost: As a healer I always have ethers, and use quite a few because I never want OOM to be the reason the party dies... even when people are taking lots of unnecessary damage. As a DPS I feel like you're expected to have very high dps (while tanks and healers are just expected not to screw up) so as a dps I'm much more likely to do everything I can to min/max (food, potions, materia - spare no epense!). HOWEVER, DPS can get away with being leeches through a lot of content.
Blame: many encounters the person who screws up is the one who causes the wipe... it's a just bit more obvious when it's the tank or healer.
I also think it's a fair point that it isn't just the "problems" it's the fact that there aren't reasons to want to tank, other than the instaque - which might only feed the problem (it's easy to level a tank and be done with it).
