I think perhaps part of the problem is people are panicking when they get defile
Maybe it's an experience thing, but the thing to remember is:
Defile is a bomb mechanic. it is a brutal one, with a twist, but the fact is it is still a bomb mechanic, just as has been seen in Molten Core with Baron Geddon, in Tempest Keep with High Astromancer Solarian, in Sunwell Plateau with the Eredar Twins, in Ulduar with XT 002 Decontstructor, in Ulduar with Freya (Nature's Fury from the Ancient Conservator)
People HAVE seen this type of mechanic, even if you didn't raid in TBC, the difference is that defile has a large radius, and a short cast time, very similar to Conflagerate on the Eredar Twins in Sunwell.
Don't think of it as defile, just think of it as a bomb which you need to get away from the raid, just like gravity bombs on XT hard mode. Sure, it is less forgiving, and if you mess up you WILL cause a wipe, which is where it is different from the ulduar examples, but similar to Solarian and Eredar Twins.
On Twins we had our entire raiding group set up boss mods to shout a massive AGGRO warning on their screen, in enormous letters if they got targetted. Here's a little background:
CONFLAGERATE - A debuff placed on a target dealing 1600 Fire damage to everyone within 8 yards and also causes them to gain the debuff as well. In addition, it confounds the target for 6 seconds and scorches players within 8 yards for 1600 Fire damage each second
This was a deal breaker - if you were too near the raid, it would daze everyone it hit, similar to a hunter scatter shot, for 6 seconds, constantly pusling fire damage to everyone - wipe
It's similarities with Defile exist in the way it was applied. She would target a player, emote, then cast it (was a 2 second cast iirc). That player couldn't wait until the cast bar came up - that was too slow and would cause a wipe. They would need to know simply when she was TARGETTING them. Hence why we setup boss mods to alert you when you are being targetted by the boss. It was irritating for tanks, but for conflag targets, it was perfect. You would see "AGGRO" on your screen, you would KNOW conflag was coming, you would start running and be abit away from the raid before she even started casting
It is this kind of raid awareness which is required on the Lich King. This isn't Naxx anymore - this fight is HARD, and requires a very high level of awareness from every single player. If you have problems handling defile personally, then I suggest playing with your UI abit, try and make it as obvious as possible when you are being targetted, just to give yourself abit of an edge - you shouldn't have to rely on other players shouting "OH GOD OH GOD", you should know defile is on you, and you should do your best to move and drop it safely away from the raid
For a little light heartedness, here is an example of how dangerous conflag was back in Sunwell - I am sure you will be able to spot similarities with defile in terms of how quickly one player can kill everyone, its funny because its Nihilum too, some bad language contained within: