Well there are plenty of MMO's i have played casually.
WoW- probably the most polished MMO out there with plenty of people to quest with of ask for help in game. Classes are diverse and leveling up doesn't take as long as other MMO's out there.
Age of Conan- easily the most up and coming MMO, with a new man behind the reins of this game the future looks promising for this title. It does require a bit more of PC power to run. but if you can handle it, it has one of the best leveling experiences I have ever had. I went form 1-80 solo playing casually and loved every minute of it.
Warhammer- A good contender for an up and coming MMO title, that's prime attraction is its PvP centered design. You can level by playing PvP matches, doing public quests, or soling regular quests. The one thing that grabbed me with this game is it allows you to jump straight into PvP at level 1.
all of these games require a monthly subscription, but other than guild wars no free game is worth your time. I would suggest WoW mainly because it is the smoothest game of the 3, and is easily the best choice for solo casual play.
If you want you can create a player on the server me and my family play's on if your looking for people to casually play with.