Insta kill or 1 shot mechanics are fun when done in moderation. Anyone that did Karazhan will definitely remember Aran's Flame Wreath (http://www.youtube.com/watch?v=717CoVJ687M), brings back good memories . The difference though is that such mechanics were rare enough for people to take notice when it did happen whereas it is hugely overdone in ffxiv IMO. Take T7 for example, is it really necessary to have 4 insta kill mechanics in just 1 fight? (voice, shriek, petrifaction, renauld)?

I feel that for a lot of fights in coil, it is less about being good, but more of not being bad. You can be the best player in the world, but if one of your teammates stones the raid its a wipe. There's absolutely nothing you can do except watch helplessly as your team falls one after another. Even if the argument for gear invalidation is ignored, individual skill should play a more important part in determining if you can recover from a mistake. Just take T7 as an example, assuming we change cursed voice to reducing the hp of stoned individual to 1 instead of stoning that player, an extremely reactive whm with lightning reflexes can benediction a tank and that's where individual skill comes in to neutralize the mistakes of others.

Such mechanics also exacerbate fragmentation among the community. It leads to zero tolerance policy among players. I have personally witnessed it in all 3 statics that I've been in, as people play the finger pointing blame game. It makes the person who made the mistake feel bad when they are singled out, and if you are the one that made it, you will feel terrible because you caused everyone to waste the last 5-10 minutes of their time.

Thirdly, scripted mechanics are actually related very closely to insta kill mechanics if you think about it more deeply. If game mechanics are designed to kill you in 1 hit, there is absolutely no room for rng.. For example in T6, you absolutely need the boss to cast devour after honey glaze to remove the mark from the player. When all fights become like this, it becomes more of a game of memorization than a game of execution. I will be the first to admit that and totally agree that if you can execute, there is no need to memorize. But the longevity/replayability of a fight is compromised heavily when boss mechanics are set in stone. Just take a look at a guide for T9 as an example:

Fight start 00:00
Ravensclaw 00:06
Ravensclaw 00:17
Stardust 00:19
Ravensbeak 00:28
Red fragment hits 00:31
Ravensbeak hits 00:31
Raven Dive party 00:37
Iron Chariot 00:38
Thermionic Beam 00:43
Raven's Ascent 00:43
Raven Dive 00:45

And this goes all the way till 3 minutes into a fight. Imagine if the same abilities were retained, but they could be cast in a random order, within certain boundaries. It would not only force people to pay attention but also keep things interesting for a long time. It's a little like Diablo 2 where everytime you load a map its randomly generated, so every time you run the map its a different experience.

Now I'm definitely not saying that instant kill mechanics should be removed totally. Difficulty does not have to come from going from 100%-->0%. I hate to use Wow again as an example, but fights like Yogg Saron in Ulduar and Hex Lord in Zul' Aman were difficult and fun without being "All or nothing". Mix things around, have pvp fights, tank and spank fights, aoe fights, debuff fights and even 1 shot fights. Even if fights become harder due to more randomization, so what? I for one would embrace it.