I was in a pretty committed raiding guild in classic back then, we were challenging for server first kills. Got alliance first C'Thun kill which felt pretty special. By the end of Naxx I just burned out, having played non stop from release. Wiping on 4 horsemen again and again didn't help .
I came back for BC for a while but lots of other guildies left as well and the magic was gone. Also I feared that I could get addicted again so I left for good. Years later I played wotlk on a private server which I liked and did some icc raiding, good fun. But I like the high population on Nost so I think I will stay here .