Keep the mob on you and survive
That's the simplest explanation on tanks. The rest is for additionnal flavor, and you could easily do the same for healers and DPS, if you like.
(Manage your enmity, put that buff/debuff, dodge AoE, etc...)
And, most importantly, "maximizing your damage output" is not a core part of the job. Before 3.0, people didn't even care about it.
Now, you could add that part to any role, tanks and healers alike...
For me, Gordias Savage really rotted this game's meta...
