I loved vanilla WoW, I was lucky enough to raid Molten core and Black Wing Lair weekly, and it was great fun as the master puller or our merry band of 40 players (sounds a bit risque when I pit it like that),
Burning crusade was fun, long lasting but turned all of my armor and weapons power ranger pink and glowing!
Lich King was great, Like you say, the ever present story line was awesome, and there was a lot of content. Cata just didn't deliver for me. Weak story, and far far to easy to get to max level. Its like the timing was all off, and as by then I was a very casual non guilded player, how the hell did I manage to get to max level in a week? It was just terrible value for money as a non raider. Cata was the reason I stopped playing for over a year.
I am loving Pandaria. It's early days, but for me its everything that Cata was not. There is so much more to do, the talent changes are a massive shift, but are absolutely bloody fantastic and add so much more to the game. Pet battles are fun, the environments are simply stunning and best of all, I feel like a heroic adventurer in a strange new land. They have put the role playing back into MMORPG for me anyway.