The thing that makes this particular MMORPG somewhat unique is that it is designed for 'party' play in mind. Most games let you experience most of the game play as solo players, taking longer than those who team up, but not otherwise preventing you from enjoying the game on your own. Not so FFXI. Out of the currently possible 75 levels of character/job advancement, you can only really advance to ~level 13 or so on your own. Monsters that are of lower difficulty give lower experience points to a point, but eventually there's a level at which you get NO experience points for beating the easy monster. And at higher levels, some of these 'easy' monsters are still going to kill a lone player.
There are other aspects of the game in addition to the experience point system that strongly encourages social play, such as the concept of 'linkshells', and most of the interesting quests and missions that make up the game.