What it means to play a tank is quite apparently not all so self-evident to certain players. (Obviously I’m talking about the wonderful bronzies I’m stuck with for the time being.) A tank is not defined by its item set. You can have all the armor in the world, but if you don’t act like a tank you’ll be just an useless XP hog on your team.
It’s actually quite similar to the job of the support. You must make sure that the enemy team doesn’t kill your carries. Roughly put, the support’s job is that the enemy team can’t kill the carries, while the tank’s job is that they won’t.