The main force keeping people playing MMOs is community. If you're on often enough that your corp(guild)-mates get to know you and care to do things with you, then it will be fun. One major attraction for the casual player over other MMOs is that skillpoint(experience points, in very rough terms) gain is in real time, meaning as long as you can get on for a few minutes each day to refill your training queue (or if you put long training time skills in when you can't), your physical absence does not negatively affect your in-game abilities compared to the hardcore gamer. Of course, your knowledge and instinct will probably still suffer.