Oh, Warcraft ... I am totally and utterly addicted to it at the moment. It's excellent - easy to pick up the basics, a good community of players and compelling. And beautiful to look at, too.
I suspect the reason it's so addictive is that it's constantly setting lots of easily achievable goals that overlap - for example: "OK, it's getting on for bedtime, so I'll head gently back to the inn to log off and en route if I kill x monsters I'll finish that quest so I'll just turn that quest in ... and oh, if I kill another x monsters by going the longer way round I'll level up, then I could before I log off go to Ironforge, put some of my stuff up for auction, pick up the new spells for my new level ..."
see what I mean? Give it a whirl, it's excellent. But expect to irritate the life out of your partner!