Knowing how to do something and screwing it up is not mutually exclusive. People aren't perfect and mistakes will happen. To me it sounds like that tank was having a bad day. It happens.
I do agree with you about people who aren't willing to learn. The question though is what can a game developer do to force people to learn, even if said people don't want to learn? You can't stop them from buying the game and screwing people over with their bad attitudes.
Let me list the things that I think make the fight a challenge for people like me.
1) A ton of mechanics happening all at once like aoes with dice and thundercloud. Especially if I am also trying to heal the tank but my oGCD are on cd and so I cant cast a quick spell to heal them. Or I am trying to get Medica II back up but i have to keep moving and I don't have an available quickcast. this is particularly difficult near the last 10 percent of Susano's health bar.
2) The purple marker for the lightning when you have to dash across to the other side. It's too small and I can barely see it past the other stuff that is going on. I have to be constantly looking at the party to make sure hp levels are good and no one is dying and the battlefield making sure I don't have the stupid marker on.
3) Same with the dice, I cant see the marker until almost too late. When grouped together with other people, it cant be seen very well.
4) Tank busters. These come out very fast and I don't see them coming because I'm not looking at the cast bar when they pop up because I'm paying attention to health levels. It's usually when I'm dpsing is when I usually see it coming.
5) I understand that most fights are scripted but I cannot memorize all of the events that are going to happen in the fight and when they occur at what minute or second. I would write them down but I don't have time to look down from the screen for a second before someone is eating an attack that needs healed through and if I do have a moment, I'm not going to be looking down because everyone expects me to be dpsing at that point.
Those are the main issues, if I think of more I will let you know. Yes I know the mechanics but it doesn't make the fight any easier.