I was an early adopter, joining the game in the Vanilla WoW beta. I ran one of the top 5 Alliance raiding guilds on the Bronzebeard server in Vanilla (Exiled Kingdom) until politics and my burn out of having the second job of running a guild killed it. I played regularly until 2009, when I returned to school.
WoW Vanilla was awesome.
The Burning Crusade was okay.
Wrath of the Lich King was epic.
Cataclysm was the beginning of the end.
Mists of Pandaria was the core collapse of the game... I'm still trying to decide if it's a black hole or a neutron star.
Killing theorycrafting by revamping the trees, introducing Pokemon, making Pandarians as a race... It has all contributed to breaking the back of the game. I left WoW and have not been back since. I moved on to Star Wars: The Old Republic and I've not looked back.