My husband recently let his WoW membership expire. He did this once before after five straight years of playing, but one of his friends (IRL and on WoW) persuaded him to rejoin and try Cataclysm, the latest version. He did, and he enjoyed it for a while, but he chose not to continue because, to paraphrase his explanation, he's neither into the hard-core guild work like he was in the past, nor is he into the casual encounters that less intense players do. He wants to be able to do 10-man raids, but most of his former guild mates are in new guilds and already committed to 25-man missions, and picking up random people to do quests just isn't his thing anymore.
The interesting thing has been our children's reaction to their dad's decision. They've been very concerned. What will Dad play now? What will happen to Jhaerith? For them, part of Dad's identity or personality is interwoven with being a WoW player. My husband doesn't regret the time he spent on WoW, but he feels now is the time to move on. He had an artist do an illustration of his main character, so he has that as well as many fond memories and stories of the adventures he's had. When his friends visit, they often talk about different quests they undertook. His account hasn't been deleted, so he can always return to WoW if he chooses - I suspect my children hope he does, because for them, that's part of who Daddy is.