When I was a kid, I spent summers in Washington State, we lived right across the street from Wildcat lake, one day I was running through the forest and kicked a hornets nest.
I will never forget that day, and still to this day have an irrational fear of flying stinging insects, especially hornets and yellow jackets.
Turbine, if you thought some of the melee characters were upset about insta killing, I guess you grossly underestimated the raw number of people who play this game just to play their casters. See in DnD, Casters are the pre-eminate class, people have very fond memories of their casters, and that day they could finally cast ultra powerful spells, and essentially became demi-gods in the process.
Back when Epic blocked insta killing altogether, your game attendance, opposition to epic questing, and prospects for the future all looked grim.
Guess what you just did Turbine, you kicked the hornets nest.
Welcome to your own little personal hell, it wont get any easier from here.
My suggestion?
Change it back ASAP, and think about using creative AI, Death Ward casting mobs, etc to solve this "perceived" problem, at least then if a caster wants to try to sneak up to the cleric and insta kill it, he can, and then go on about his merry way.
Just some friendly advice from some one who has a pretty good pulse on the game.