Originally Posted by Xypherous
A Tank's fundamental purpose is to create openings for your team to score damage.
Either you provide strong initiation power (Malphite's Ult) to start a fight so that your carry has that opening to attack...
Or you provide strong counter-initation power (peeling) to stop the enemy invaders from reaching your carry... again giving the carry a safe opening to attack (time to kill the invaders.)
This is also why they have typically have threat over time abilities - because they need to present a large enough threat over time to give your carry's time to burn the opponents down - either through damage, CC, or some kind of awful debuff.
And this is also why they have burst mitigation - a tank that dies in a few seconds doesn't create enough of an opportunity for your team to *do* anything with those seconds - a tank wants to, paradoxically, take aggro but be immune to enemy attacks for as long as possible.
For example, Blitzcrank isn't quite a tank - his initiation isn't that reliable or strong, and his peel isn't that great.
Blitzcrank + Frozen Mallet + Frozen Shroud is a beast of a peel for your carry though - no one's touching your carry if there's a Blitzcrank with Frozen Mallet + Frozen Shroud. That Blitzcrank with Frozen Mallet + Shroud standing next to a Vayne? No one's going to be able to lay a hand on Vayne, giving Vayne the time to DPS target's down.
Peel isn't the only thing that should define a tank, it's a combination of the traits you named.
According to your Blitzcrank example Janna would be one hell of a tank because no one will ever reach your carry if she doesn't want it. Her ultimate, the Q and the slow from W give Vayne or Kog'Maw enough time to kill 2-3 targets all by themself.
A full, true tank has at least two of the following:
- ability to soak burst-dmg and to some degree sustained dmg (no champion soaks late-game carry dmg for more than a few seconds)
- peel enemies from the carry
- the ability to initiate a fight
- ability to force the enemy to attack them (either a taunt-like ability or high potential dmg)
- ability to disrupt
Example for true tanks:
Nautilus has strong initiation and strong peel. His (innate) dmg-mitigation isn't that great but if ignored he will do good dmg over time.
Rammus has good initiation and powerful dmg-mitigation along with good dmg over time and a tool to force his enemy to attack him.
Amumu has very strong initiation and good dmg over time however weak (innate) dmg-mitigation and no real peel (either initiate with Q or peel with it, not bot).
Sejuani is a tank in theory but after her ult you can basically either ignore her or blow her up because she has no dmg or peel and no dmg mitigation.
Funny thing is that this would make tanky-dps real tanks as well because in case of Irelia or Wukong or Shyvana you have all the reason to attack them (they jump to your carry and kill him/her) and they have good ways to mitigate dmg (free resist stats) but since they have to itemize for dmg they can't take too much burst...is what I'd like to say but let's face it. Irelia and Shyvana can take almost as much dmg as tanks while blowing up carries because they get items that provide both offense and defense.......
Anyway they are no real tanks because they can't protect their carries directly and often lack powerful initiation.
(Yes, I think the class "tanky dps" as it is now is not that well thought out...Irelia.......)