Mag is the culmination of t4, he takes all aspects of the learning curve and puts them together to be the first true test of all aspects of the raid. Lets face it Kara is a very well designed raiding tutorial that gives you the basics one encounter at a time all while giving you the gear necessary to progress to the first 25 man encounter, Gruul is nothing but a gear check to see if you are geared to progress 25 man and beyond.
Magtheridon brings it all together and is the first real 'raid boss', the pinnacle of t4. He is not a test of gear, he is not a one trick pony. He is the first skill test, and looking back not a magnificently hard one, really, handle some adds, click a cube on rotation, transition through a raid wide aoe bamo dead boss. Trained monkeys could do it.
He was also the gateway into TK. But they removed the attunment and people started skipping him because they could never quite get him. Those same people coasted through Lewt Reaver, and the easy bosses in SSC/TK but got absolutely stonewalled on the tougher bosses, notably Vashj/Kael.
Tier 5 bosses all got nerfed to hell save Vashj/Kael. In soem cases with good reason (Hydross) in others? Not so much.
I suppose RoS, Bloodboil and maybe Gorefiend will follow soon.
Magtheridon, nerfed, was never hard, though. He wasn't really a learning lesson for Kael and Vashj - the fights are wholly dissimilar, though I guess Mag helps weed out reatrds. Old Kael took ages to learn no matter what, but was not in himself that hard.
And now we have the removal of T6 attunement, which doesn't bother me at all. Why? Because the guilds who never learned what they should have on Mag and failed because of it when they got to the end of t5 are going to find out the hard way that t6 is just more of the same. Couple easy bosses, some not so easy trash and then bosses that are to Vashj as Vashj was to Mag.
It is only a matter of time before the same old story repeats itself, or the nerfs to Archimonde and beyonds starts to happen.