I forget where I heard it, but I remember something about how Arenanet wanted the combat in dungeons to sort of inhabit that space in a traditional trinity MMO when the shit hits the fan, where a tank or healer dies and the rest of the party is having to kite, and use CDs, and heal, and do more than just stand there going through their dps rotation. Like the last 2% on a tough progression boss in WoW when the tank goes down, and the hunters are having to hit distracting shot and pop deterrence, healers are having to throw up DoTs, and everyone who can is having to taunt the boss back and forth while squeezing out what DPS they can. Obviously in GW2 this won't just be a few seconds of "oh shit" before a wipe or a skin-of-your-teeth kill, but will be sustainable over the course of the encounter. The professions are designed to have this utility, and to coordinate in such a way.
I know this doesn't really add anything new to the discussion, but I remember that idea sticking with me, that the dungeon combat in GW2 is going to live in that reactive, dynamic space most trinity systems only encounter if shit goes sideways.