The basic theme I'm seeing repeated in this thread, like any other tank discussion thread, is that *just about* any job can tank... when geared/subbed right and the monsters are appropriate. Which, I can totally agree with. There is so much unused equipment laying around which could effectively enhance a Warrior or Monk to decent tanking status, hell, even Samurai really. But they go unused, because until now, there were always PLD's and NIN's lying around waiting to be snatched up for parties, so they just stacked as much DD gear on since other jobs were available for the role.
Well, things have changed, since bringing WHM and MNK up through the ranks recently, I have only partied with a NIN or PLD maybe twice each, the rest of the times we improvised a tank, relying heavily on Dancer/Ninja jobs and Warriors who we had to FORCE to trade hate between each other. It wasn't always pretty, but the jobs got done. And I gotta say, the best tank I've seen so far has been a SAM/WAR slashing his way through our mobs in Garliage. The dude held hate, and did some great DD'ing to boot. And we easily kept him alive, so there ARE alternative tanks available, it really has to do with the gearing of the job. The Samurai mentioned above was asked to tank before the party started, and happened to have some heavy armour availale, along with VIT+ rings and a few other goodies to help with the job role. All it took was a leap of faith by the other party members to give it a shot.
That being said, I would love for SE to create another tank job alternative, one that belongs in that role. What it could be, I have no clue, so by all means speculate on. That's most of the fun, right?
And in response to the whole "RDM can tank if... etc" discussion: You can transform most any job by simply giving them one more job ability. Saying "RDM could tank if they had this or that" is a completely viable arguement, obviously, as that arguement could hold water with most other jobs as well.
But Red Mage is still a Mage, and in keeping with the themes set by Final Fantasy, should never assume a tanking role. As in what Baj said, if you change the job that much to allow it to tank, it is no longer a Red Mage by SE's standards. Just my humble opinion.