My guild never cared much about Undying and similar achievements when they were current content, because we've just never been big on raid achievements. Just getting the bosses down has always been the priority.
A few months ago we finally did an Undying run since the option to earn that title is going away soon, which I tanked on my paladin at the time because we were short on tanks. Since then we've occasionally tried to get another run together to get the achievement for those that missed out on it before (like my main), and it's been nothing but a major pain in the arse. Tonight was our third failed attempt in a row.
Whether someone pulls aggro on Patchwerk or stands in a shadow fissure on Kel'thuzad, there's always something, and it's slowly driving me batty. We have three times as much health as we had when we first ran this content, do six times as much damage, and still people can't get the basics right? I'm not someone who tends to get angry easily, but it's hard not to feel a certain seething rage when this kind of thing happens over and over again. After all that person's death just completely wasted an hour's worth of effort.
The Ulduar version where you don't have to get it all right in a single lockout is a lot more forgiving, but it's still infuriating when you only need one more boss in the whole instance (usually near the end) and that ends up being the one where someone messes up, once again by doing something really dumb like standing in Mimiron's shock blast or pulling aggro on Vezax. I actually managed to get my own Champion of Ulduar title last night (after someone had messed it up the week before), but this time it got borked up for someone else who still needed a different boss.
And then, on top of feeling angry, I feel guilty for feeling angry. Because, at the end of the day, I like my guildies, and everyone makes mistakes sometimes. It just feels wrong to get that mad over them dying a single time. It happens to everyone sometimes, right? Too bad that it happens to them when the whole effort of the night hinges on nobody dying... which just makes me hate the whole achievement in the end.
In a way this is kind of weird, because achievements that reward you for playing well and getting it just right have always been among those that I approved of the most. After all I loved the Zul'Aman bear run, and we spent a lot of time messing that one up as well. I suppose the main differences between that and something like The Undying are:
1) Since nobody ever got to do a bear run while outgearing the instance by multiple tiers, it wasn't as utterly humiliating to mess up, and some of the loot from the other three chests was still useful to people even if you didn't get the fourth one.
2) People were generally better at watching their threat. During this expansion even a lot of dpsers that I value a lot seem to have completely unlearned how to watch their aggro, because it simply didn't matter most of the time. I'm hoping they'll get back into the groove come Cataclysm, because having a whole run turn out to be pointless due to someone pulling aggro is just stupid.
3) As tough as it was, the Zul'Aman bear run still allowed for a bit of leeway and compensating. I remember on the run when we got our first bear we actually had two or three deaths, but managed to make up for those precious seconds that we lost while resurrecting people. There weren't really that many insta-gib mechanics either, where a person standing in the wrong place would die instantly. (Maybe Jan'alai's fire bombs...)
People often like to rant about having to carry bad players and I agree that it can severely affect your fun when you have to compensate for someone playing truly terribly, but not being able to compensate at all, completely having to rely on everyone else to not stand in the wrong place and to not pull aggro on the untauntable boss or else your entire evening is ruined... that's even worse in my opinion.