Out of all the forum posts written concerning raid content back in Burning Crusade, I remain fondest of a guide to Tier 6 written by Gragnarth of Andorhal, who I assume must be a deeply cynical person by nature. Within you'll find expert tips on Illidan ("If YOU get demoned you say something to the effect hey I got demoned, and then hope that you are well liked"), Shade of Akama ("I play a fury warrior, and as a result i have no clue what the strategy for this boss is"), and Rage Winterchill ("Make sure you have at least 1 person bandaging the Main Tank every minute"). But the comment that seemed to get the most mileage was one concerning Illidari Council, which was colorfully described as "THE SUPER BOWL OF NOT STANDING IN THINGS!" This phrase subsequently entered the parlance of many a raiding guild, and I'm reminded of it whenever I look at AoE-intensive fights.
Kel'Thuzad isn't a fight with the kind of AoE damage you'll see on (for example) Malygos, but I rather like to think of him as being the Super Bowl of spreading out. For every time you've heard your raid leader howl at the raid to "Spread the ^*#% out!" before, you'll be hearing it five times more here, and with good reason.