I'm sorry but you guys talking about the challenges in vanilla don't know what is actually challenging. Buffing 40 people every 15 minutes isn't a challenge - that's a shitty design choice. Taunt missing isn't a challenge, that's stupid fucking design.
Naxx40 was hard shit - I was there, I know. My neighbor was in one of the guilds from Zuljin that actually even killed KT.
None of what you guys are listing is hard. The heigen dance isn't hard, it's a retard check. Nefarian isn't hard, it was a gear check.
The encounters these days are way more complex. Even finding 20 people capable of mythic is the hardest part of running a guild for me.