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.
The tank must go in, and soak up as much damage as possible, while also dishing out as much as possible. The more they take, the less goes to the carries. The less the damage, the lower the chance that the carries die.
It always fills me with bitter amusement when a “tank” bashes the carries for dying, while said tank hides behind the carries in engagements and then walks away full health grumbling about how their team is horrible. Go in, initiate, keep the enemy team busy so your carries can kill them. A Thornmail won’t make you a tank – having the enemy shoot you wearing it will.
On the other hand, if the tank is not a threat in any way, the enemy team will just ignore them and thus their whole purpose is lost. Marksmen aren’t shooting that Leona because it’s their hobby, but because if she’s not gotten rid of, she’ll keep stunning them for seconds, which pretty much equals certain death for a squishy carry. If that enemy Darius can reach the mage, it’s the fault of the frontline (the fighters, the tank), and of course the mage will be afraid of getting dunked.
Am I wrong? Well, I’m in bronze for a reason then.