I see it all to often, especially in MoP where as soon as we outgear 5 mans they become faceroll easy. For those of us who don't raid but enjoy group activities this kind of saps some of the fun out of these runs. So what if with the current 5 man dungeons (this can transfer to WOD as well) the boss abilities don't do X damage but instead does % of max health damage.
With those abilities that are supposed to be deadly can remain deadly, the ones that are supposed to hurt a lot still do no matter what your gear is. It would reward avoiding these abilities as well as punishing those who don't. As soon as the dungeon is no longer relevant, could be a certain amount of time into an expansion, the next major patch, or even the next expansion entirely, then switch to this ability does X damage.
That way if people are leveling alts and just doing those for basic sets of gear it would be easier than when you tried to do it when it was relevant.
What are your thoughts and opinions?