I would recommend World of Warcraft if any of the following conditions apply:
- You have played other MMOs and enjoyed some of the aspects of them
- You are a social person and enjoy (or don't mind) talking to other people and working cooperatively towards a goal
- You like competition (PvP) but don't take it personally
- You don't mind somewhat repetitive gameplay broken up by learning new skills
- You like experimenting with things such as builds, skill combinations and the like
Myself, I play about one to two hours a day, usually a bit more on weekends. It doesn't "own" my life, and it's not at all difficult to play on a semi-casual basis, so long as you have friends (or better still, a Guild) who don't mind if you run along with them in dungeons for fun and the occasional gear upgrade. Plus these days there's tons of daily quests you can do for cash, from aerial bombing runs to vehicular combat to jousting.