I personally love mechanics that forces your raid to really know what they're doing such as Defile or Heigan's Safety Dance. Ulduar had a lot of interesting mechanics as well, especially Yogg-Saron. Having to turn your back to mean things in Yogg-Saron's different realms was pretty smart, you couldn't just send someone clueless in there and they'd nail it at the first attempt like many heroic bosses today. General Vezax "heroic" mechanic was interesting to deal with as well, having no mana regen really forced you to rotate healers and never overheal until you really overgeared the fight.
Nowdays I think fights are a bit too much about tossing random damage around and rotating raid cooldowns. Hard fights are usually the ones that have a lot of unavoidable damage and you simply have to rely on massive cooldowns coupled with good RNG to beat them until you outgear them. It sort of feels like Blizzard don't want to add single "hard" mechanics due to the fact that they wouldn't work at all in raid finder or flex mode, and rather resort to stacking multiple easy mechanics at the same time of which some can be ignored on easier raid modes.
Oh. And I really miss that you simply don't have to CC aside from stuns and interrupts nowdays. It's sad that you have abilities like polymorph, fear and the likes yet you're never forced to use them outside of PvP. Bring CC back to the PvE part of the game!