One thing I really appreciate about Blizzard's endgame design is that it's much more about player skill than having massive loot. Levelling up to 70 and through Karazhan, the primary driver of success is your gear. The more DPS, mana, and mitigation you have the easier you can plow through. You can outmuscle pretty much all the bosses in T4 content. The one T4 exception is Magtheridon and his crazy cubes. That's a totally artificial mechanic, but at least only a few members of your raid have to learn the gimmick.
The T5 and T6 raids are full of player skill checks. And some of them are pretty artificial and cheesy. Right now my guild is stuck on Teron Gorefiend, the first really difficult Black Temple encounter. The gimmick is a random raid member gets killed, reincarnated as a ghost, and then has to kill four Shadowy Constructs before they reach your raid. The Constructs are immune to everyone but the ghost player. And the ghost itself has completely different abilities from what most players are used to; an AoE Shackle, an uber Frostbolt, and some other skills you never use. It's a bit like the Chess event in Karazhan, only more difficult to actually win.
What irritates me about Teron is it's a totally artifical encounter that you can't really prepare for. It takes people two or three tries as a ghost to figure out how to kill their Constructs. One try to find the stupid buttons for the special abilities, one to get the timing down, and then maybe the third try you understand it well enough to win. But everytime someone fails it's a wipe. And only two people get experience with the ghosts on each attempt. So a raid of 25 people needs to wipe about 20-30 times before everyone has had the chance to learn this odd skill specific to this one encounter. There's even a simulator to help you train ahead of time; I spent an hour with it before our first raid and still had a tough time. How crazy is that? A simulation to train us to play our online simulation game.
I don't mind having a challenging fight, I just don't like an encounter that requires skills that are nothing like what I normally do. I much prefer the design of the Vashj and Kael'thas fights. They're also incredibly intricate and require at least 20 of your 25 people do the right thing at every moment. But it requires you play your class well, not play some arbitrary pet you've never seen before.