For me it was the community, me and lot of friends played it whom I know from school. As I went I started to get bored and started lots of different jobs. The only reason I stayed was because of the community.
When I first got WoW I played it for 2 weeks and stopped it, thought it was rubbish. Came back 3 months later and played it for 9 months. WoW isn't something you can do justice with for just 2 weeks, I would never go back to it though. Its good for a while with you friends (much better at highest level in a guild) but after a while its just patience and repetiton.