I played some amount in vanilla but was never in a good guild that did anything more than MC, which was really time consuming and boring (but I still liked the epicness of the old 40 man raids). When BC came out though I switched servers and found a really good fun guild. Played super hard core in BC through sunwell, had a great time. When LK came out though our guild turned casual. No one wanted to work on progression, just grind easy mode raids over and over. Game started to get real boring for me at this point, I only logged in to raid because I was the MT and guild depended on me, I wasn't really enjoying it. When they released the coliseum raid is when I pretty much quit, that shit was just phoned in by blizzard and was so boring. Also it was so easy my guild didn't need need organized raids any more and we just fell apart. At that point I quit and started using my afternoons to go to the gym instead and get back in shape.
A little before cata came out, and due to my nostalgia for the good old days of raiding in BC, I applied for a top tier guild to tank for through cata. When I logged in for the raid tryout (heroic LK), I played for about 2 hours and then quit and said "screw it, I made a mistake, I'm sick of this game. Sorry for wasting your time". And that was that. While I miss the social interaction with my old guild, I can't stand trying to start over and being the new guy in a new guild. Especially since the game itself is boring. I really only ever played WoW for the social interaction.