There is one mechanic, above all of the others in WoW that for me really just took all of the joy out of raiding, and that mechanic was the "Enrage Timer".
Think back to learning Molten Core or BWL. Back then, the gear checks were "Are your tanks equipped well enough, and your DPSers good enough to kill the boss before the healers crap out", and "have you done the encounter enough times to teach a signifigant portion of the raid attendees the ins and outs of the encounter?". That was fun. You'd spend a couple nights on farm content, gearing the less geared, then you would progress to the new bosses, spend a couple days on them (longer on stuff like domo and vale and untauntable firemaw) until people figured out what they were doing, and how the encounter was designed to be played. If there was a mistake made, and one or two of the DPSers or healers died, it wasn't a huge deal because everyone was there doing what they could.
Then came the idea of an "enrage timer". IIRC, Satura in AQ40 was the first real boss to employ the "enrage timer", but Satura's HPs were so low, and the enrage timer was so long that no guild really had any issues dealing with that. Even the twin emperors (the only other non-naxx boss to have a true enrage timer) had an enrage timer (set at 15 minutes) that was not the crucial part of the fight. I believe that our guild only really passed that mark once before we dropped them. The raids there were not limited by a certain set time that the devs could designate a minimum raid DPS (boss HP / enrage time = minimum DPS).
Patchwerk was the only fight pre-BC with a REAL enrage timer designed at testing DPS mixed with aggro management. Many guilds struggled on him because it was a relatively simple encounter that blizzard designed strictly as a "Gear Check," and it was such a different concept than any previously seen encounter. Keep the tanks up and on top of threat. Have the healers pull out all stops and heal the tanks through the HS's. Once that is done… how big are your DPSer's balls? Learning the encounter didn't even require the DPS to be there. Your DPS could have sat out and run to the blasted lands and practiced their single-target cycles versus the unkillable mobs, and have been more productive.
Even the other fights with enrage timers in naxx weren't designed around the concept of a gear check, or a max time on the encounter. Heigan's enrage timer was put in to ensure that you weren't only bringing in 10 people who could dance, and 30 people who didn't understand that green goo = bad. Thaddius was "can your folks move left or right in a coordinated manner, and are they awake enough to not miss a stupid jump?" Both of the fights were simple enough DPS and healing wise that the enrage timer wasn't really a check, but more of a byproduct of the rest of the fight.
Now we hit BC. Every second mob has an enrage timer. Even mobs that don't need it. I just don't get why they took such great ideas as making raiding about learning the encounters, and practicing a bit, and then executing, to encounters that require flawless execution where if you lose one person and have no means of recovering that person, then it is a guaranteed wipe. Where are the "learning" kills, where the boss dies from, of all things, a hunter's dot when 2/3 of the raid is dead, and then the next time only 1/3 of the raid is dead, and the next time only the dingleberries you drag through content are dead. Those fights are gone, replaced with content that just makes the experience less fun.