I'm glad that Trion included several themes in each of the classes. I do wish, however, that mages had a mage-tank. I think that it would round out the calling. The rogues seem to do O.K. with the Tank/Support/DPS combo. And clerics are strong too.
I think from what I've heard warlocks might be able to tank with a good healer watching there back.. they have pretty good party and self healing abilities.. I don't know the abilities exactly tho so I can't speak from experience.