According to ESRB, this game contains: Violence
Parents will want to think long and hard before grabbing Final Fantasy XI for the kids. It's not that the game is filled with objectionable material (although, since it is online, a lot of things in-game are dependent on your fellow players-and as Sartre said, "hell is other people"), but it's a game that requires a huge amount of time to play. Trying to juggle schoolwork, familial obligations, and this game is a difficult juggling act. To get the most out of the experience, players have to devote a fair chunk of time to playing.
Attention hardcore gamers: this is the game for you. If you're at all interested in MMORPGs, then get this game now. While it's not a flawless experience, FFXI will eat up gaming hours like Rosie O'Donnell at an all-you-can-eat buffet.
Casual Gamers on the other hand might want to sit this one out and wait for Final Fantasy XII. Not only does one have to buy a hard drive (which comes with the game on it) and a USB keyboard (because communicating is nearly impossible without one-the soft keyboard just doesn't work), but the $14 a month playing fee and time investment required make this something that's not really worth it for players who only want to dabble.
Deaf and Hard of Hearing gamers have nothing to worry about. While you'll be missing out on the game's music, all of the dialogue is in text.