I totally disagree with part of your statement. While I do not believe that BC, Wrath, or Cataclysm has ruined the current state of World of Warcraft, I really did enjoy Vanilla the most. On that note, when BC came out and we were able to see the new worlds with the amazing skyboxes, the new (well, most of them were new anyway) monster models, flying mounts--that was an awesome experience for me, and Blizzard's best time. Smaller raids were great, but tore guilds and friendships apart.

One thing that really sucked about post-vanilla gameplay, was the PvP. Anybody who PvP'd regularly would likely agree with me. Flying mounts ruined travel routes, there's virtually no more world PvP, and the organized PvP is a gigantic mess. Now, I'm not talking about the 10 hour+ AV games, those were fun and were even as disorganized as they are today.

Do you remember riding your mount from Burning Steppes or Searing Gorge only to meet up with a few stray Horde or Alliance looking to stop your raid? Do you remember looking at somebody and saying "Holy crap, he has an arcanite reaper!"? Those days are gone. Resilience is a gigantic joke, especially for players who want to start PvPing, but can't compete. It takes them 5 times as long to accumulate gear for the person who has been PvPing for nearly a decade and kept upgrading in small bits.

Wow, I went on a tangent. So here's my general view: vanilla was amazing, challenging, and fun, as was a few of the expansions. If you wanted to be casual, you could be casual. If you wanted to raid 3 nights a week for 4 hours per night to see Ragnaros die, you could be a hardcore raider.

Finishing Dragon Soul on hard mode in 2-3 hours after some intense progression?

That challenge is gone now.