For me, it's the sheer amount of stuff you can do. Combat is never boring, FF14 DESTROYS the sham that is WoW's profession system with sheer depth, there's always something you could be farming, some quest you could be doing. If I had a vacation, and all I had to do was play this game, I still doubt that I'd be able to get all the roulettes, dailies, weeklies, script farming, weekly caps, GC turn-ins and levequests done.