If standing in the fire of challenge modes kills you instantly, that's great and all, but it does not improve the (how I see it) under tuned quality of 5 man heroics. I'm not asking for bosses to have more health as all it would do is drag out an under tuned fight. What I think would be cool is more punishing mechanics if people fail to avoid them.
The way dungeons seemed to have been designed was to the idea that if you stand in bad stuff, you will eventually OOM your healer and fail. The OOM design is not working. I think the best way to keep myself from being bored doing heroics is if every fight was as punishing as Armsmaster Harlan, and then add fire puddles on the ground every 5 sec while he's doing his blades of light. These fire puddles have a 2 second arming time, and then deal 55% of your maximum health every 1.5 seconds, but wait, it gets better! The adds that get caught up with him have their armor and weapons drawn into the whirling glory that is death, and after 1 round of spinning, one of the weapons shoots out somewhere in the room that a player needs to pick up. This weapon, being light infused can now be thrown via extra action button once it is picked up. 3 second cast time on the light infused weapon, stuns Harlan for 3 seconds also stopping him from continuing blades of light.
Do that to every fight and heroics may feel heroic, even if challenge modes offer that much punishment and complexity, I feel challenge modes need to be about speed and efficiency, and all heroics of any setting need to be bottled death. ATM people can stand in Striker Ga'dok's fire trail on gate of the setting sun, while licking Windex, and still live to see the end of the fight.