NPC"s being smart enough to go after the softest targets only really works in games where you can protect said targets. To use the team fortress analogy, most people -do- go after the medic first but I can always stand behind the guy I"m healing or hide out of sight, stuff like that.
In most MMO"s, first off you"d need viable collision detection. And at that point all we"d do is corner the npc so he can"t get around the tank and be done with it.
I agree as a mechanic it needs to eventually evolve, but I don"t think we"re quite there yet. I mean to be honest, if 99% of these NPC"s were smart as soon as they saw us coming they"d run for backup. It"s like evading NPC"s in WoW. Shouldn"t they be smart enough to go find a couple other NPC"s with ranged attacks and own the shit out of our exploiting asses?
Ah well...
I think removing heals would be a pretty neat first step though. You look at Reliquery phase 1 where you cannot heal and you get a general idea of what I was thinking of in Zehn the MMO. Your "real tanks" try to take the bulk of the damage but if you want to survive you need to learn to find ways to prevent it and spread the damage around.
You kind of avoid the silly scenario where Illidan doesn"t just go, "You know what? Fuck this." and walks around one-shotting your healers.