Ok, nervousness in ICC, that's a different thing.
There are spots that are tricky and most of them involve spawning adds.
For some tanks tanking the left side adds of LDW can be horror. Well, only if you can't keep your cool. In many cases, this is the perfect storm. You have the two melee adds that spawn quite a bit away from each other and the caster add in the middle. The melees cleave. Hard. Most of the DPS assisting yo is melee themselves. So you need to position them fast or you will have dead rogues. You can't really practice this and the dead rogue WILL be slightly annoyed.
Tell the dead rogue to die in a fire. You are not supposed to handle something like this alone. Our melees greet the middle caster add in the alcove with face stabbery. While they are occupied you can grab the add closest to LDW and taunt or whatever the melee add furthes away from LDW. It's easy to spot. It's running towards your healers. Once you have their attention, you position them. Now it's DPS problem not to stand in the cleave.
If, by contrast, you tried to grab all 3 adds you will lose one and possibly have dead rogues since you can't control fast enough where the cleaves go.
Last weekend I tanked in a very, very fresh, very, very PUG ICC25. Me as a warrior, tanked the right side. Cakewalk. We had a bare durid iz tanks on the left side. Quite often I had to pick up one of his adds making a beeline to the middle.
Was he bad?
Did he fail?
...there was only one melee assisting him even tho we had assigned 3. So 2 melees failed. We had 8 corpses and LDW died just the moment she enraged(I had even forgotten that she had an enrage). Let's just say, after this first run we weeded out half of the raid since the rest was even worse.
The next annoying thing is staying alive on Fester. It's amazingly hard in 25 man when you are not evenly geared. Evenly geared as in: "I don't really need anything from this instance anymore apart from that elusive drop. WTH am I doing here?". So if you do not outgear that encounter, you need to plan your CDs. Have you macroed shield block to shield slam? Now is the time to uncouple them. You need this during the second inhale just before you taunt. You have never used indestructible potions? Start now. Trust me. You always wondered what the Glyph of Indom was good for(lousy ilvl245 badge trinket)? This is what it was built for.
Where do you think all this wisdom comes from? That's been trial and error by other tanks. Mostly error, I guess. You can't just learn from reading guides or watching movies. You need to fail. Repeat with me: failing is good. Just learn something from it. Believe me, DPS and healers screw up all the time. At least twice a minute. So you screw up once and get blown to bits? Weeeeell. Happens. Only take the blame when you are sure you screwed up. And fess up when you KNOW you did something wrong. I once taunted Fester while I still had 9 stack up. Uuuups. Big badaboom. Wipe, corpserun/jokes in vent, kill, loot. End of.
You are not in this alone. And there are 24 other people who have to assist you.
It's not only that you know the pulls. It's also that DPS know where to hit. Hard.
It's been said before. Practice.