Raiding is a good judge of a person, you and twenty four other people raid an instance together, moving through trash fights, and eventually to a boss designed around twenty five people, fully flasked and using consumables performing to their utmost to achieve the necessary DPS to down the boss before it enrages and make the Tank feel very squishy. These fights can often be approached in a small-group format and doing so often really emphasises how dependent and important the rest of the group is to you, its impossible to Tank a raid boss without a healer for example, the damage input is tuned to a level such that there is simply no way to survive outside of a team. Certain fights add in the aspect of personal responsibility, tht is where every member of the raid must take responsibility for their own survival, for gearing and using consumables and cooldowns to the maximum effect otherwise the penalty is applied to the whole raid in a more stringent way than simply the loss of that player. Magtheridon is really the first fight you will encounter with this aspect outside of the Tanks, that is to say that the cube clickers (20% of the raid, pre-nerf 40% of the raid)had to take control of and be responsible for controlling the blastwave, anyone that failed inflicted grevious damage on the raid. This level of personal responsibility though is easily shouldered by your most confident and capable raiders, the fight itself does not stress you beyond this aspect.
There are however fights later in the game that take this principle forwards, they make the 20% or so of the raid that needs to shoulder the responsibility random, or make the responsibility raid wide. This aspect lets you see who your raiding partners really are, who is capable, who is interested but failing in some way, and who is simply slacking. Lets look at the three major personal responsibility fights in tier 6 content, Gurtogg Bloodboil, Teron Gorefiend and Archimonde.
Gurtogg is a really easy fight in principle, three tanks trade aggro to stop a stacking debuff reducing their armour to 0 and the DoT stack killing them, three groups of five people move to be the furthest out to accept a DoT which again stacks (thus the three rotating groups), and every minute one person is randomly selected and must be spam healed to survive a mini-enrage (you get boosted armour and health). In short eighteen people in the raid have a raid role they must ensure they survive otherwise the initial plan falls apart, but its survivable. The enrage however is a random target, 100% of the raid has the responsibility of doing their best to not die (pots, shields, self heals, trinkets etc), the only people you really want enraged on are the Tanks because it simply isn't a risk to them, its designed to kill clothies and non-Tanks.
What this really shows is your raid reactions and your ability to react under pressure to a random event, if you aren't doing your utmost to survive and die, the enrage will pretty much kill anyone else he reaches except through a Warrior Shield Wall.
96% of the raid (24/25 people, the Tank is not a valid target) risks getting a debuff, after 45s you die and respawn as a ghost and must kill four constructs before they reach your raid, oh and the constructs can only be killed by people in ghost form, oh and the ghosts despawn, the constructs don't. Teron is a good fight, anyone bar the Tank (and if they aren't competent why are you letting them tank?) must be capable and know enough to kill their constructs, one or two at a time in the raid will tend to push your healers, three or more tends to wipe the raid. Again this is a really simple fight, DPS Teron, kill constructs, yet it causes great problems to many people and to many raids. I have been picked twice, the first time I ran to the corner and killed them easily, they didn't get near Teron or the raid, the second time with 30s left before I died I was killed by a construct in the raid, and immediately became a ghost spawning 4 more constructs in the raid. As I had shown the first time through though (and I never beat the simulator) it was simply a case of doing your job and hitting keys, pretty much 5,4,3 tab, 3 tab... repeat every 15s.
Archimonde the Defiler:
Giant boss stand in the middle, and Doomfires spawn, run away from the fires (since "Stay out of the fire" is the tag-line of TBC its a fair guess fire is bad), if you get thrown in the air use the really cool Tears of the Goddess just before you hit the ground and you survive, and if you can decurse anyone that is near you and on Decursive. Sounds simple right, its not a DPS race at all, raiders at this level can actually run away for half the fight or more (one of our top Warlocks could go afk for 6 minutes and still easily meet his DPS requirement, heck the Tank can make the DPS requirement!), however if you don't manage the not-dying part he gains Soul Charges. Each Soul Charge hits the raid for ~5,000 damage and knocks you down (remember the running away bit), in short anyone dying can start a chain reaction wiping the raid, yet not dying is pretty easy, there are of course cases you use your tears under the minimum safe distance but barring those its a "run away and survive" fight. Archimonde however offers guilds the chance of 20, 40, 80 or more wipes to get it right, in fact one of the best ways to down him is to go in with less than a full raid if people keep dying, yeah a T6 end-game boss doesn't need a full raid to kill.
... And the point is?
The point is simple, most fights in the game are positively marked, good things happen if you do your job and do it well, but if you mess up there is very little beyond the loss of you to the raid that is a penalty, the fights I noted above are negatively marked, doing your job gets you zero points, doing more gets you a kill in under the enrage, and screwing up penalises the raid.
This is something that the game needs more of, and to introduce it in a better manner, Blizzard assumes a level of knowledge in the game and a level of skill, dropping people into fights like these without the confidence and skill to take that personal responsibility simply results in heartache and wipes. These bosses are in the end game, they should not be easy, however the number of players who take for granted that they will not need to perform, or simply don't is large, and seems to grow, with Wrath of the Liche King Blizzard appears to have introduced limits on consumables and threat, this means that you will need to take more responsibility, no longer is a DPS class simply a DPS class, but a mana fountain, a threat reducer or something more, the raid will live or die based on the ability of its raiders to take their roles seriously and perform.