I wouldn't say it's the greatest MMO ever but it's the only one that kept my interest for long. I am not sure why, I think a big part of that was the way the game would force you to make friends when I started and got me staying for those friends I made.
The differences I noted from the other MMOs I tried were:
-The community, somewhat people are more friendly in FFXI.
-Being able to have multiple jobs on 1 character. As much as I liked wow, having to create new characters to try a new job kinda killed it for me.
-The complexity of the game. Each spells having their own stats for example, leads you to need more gears compare to most MMOs where you just get the highest ilevel set and you're done.
-The lore. I hated doing quest, but those we're challenging and would take you quite some time to complete. You would feel a certain feeling of accomplishment when you done them. Most MMOs have very simple and easy quest.
-The open areas, I hate arenas/dungeons. I liked exping on random monsters out in the open, it gives you a lot of freedom.
-The need to make friend to accomplish stuffs, no dungeon finder/raid finder/party finder ect. Would make you have to communicate and learn to be more nice toward people if you wanted to advance. Most MMO nowadays, people can be *** and still accomplish stuffs because they will use an auto party system.
-Skill means more than gears. That doesn't really apply to today's FFXI but it used to pre abyssea era. If you were capable of playing your job properly, it could balance your lack of equipment.
I think to me FFXI will always be no.1 until it dies than maybe, I will try to become a somewhat responsible adult or something xD