Yup, Del covered it pretty well. It's the immersion, the absolute openness. The way the races and nations interact in what seems like real time.

There's also the way the social element of the game adds to the story. You hear other people talking about going through such-and-such section of the game, fighting such-and-such boss, and you get excited, because they're excited, so you start to imagine it for yourself, and that builds and builds until you get to experience it yourself.

I think the blistering difficulty of the game helped too. It was a detriment to the game in some ways but also one of its greatest strengths. Some missions were quite time consuming, and the fact that you could lose quite easily, meant you had to really focus, which ratcheted up the intensity. Not to mention the fact that most bosses required a length tense trip through monster-infested passages so by the time you even get to the fight, you're tired and stressed, a little slap-happy, but if you can pull it off, it's all worth it and you feel an intense sense of accomplishment.

A good story unfolds. All the elements are there in essence at the beginning, in the fabric of the world, but in a tangle so you can't see how all the parts connect. You begin to unravel the story and things fall into place. For instance there were landmarks built into the landscape that were visually interesting when you first come across them, but had nothing to do with anything as far as you could tell. But later you find out that this ravine you passed by a hundred times was actually created during a great war, or that there used to be a city where this beach now sits, or that this monster used to be one of your own (an early boss in the game, the Shadowlord, is a monstrous hulk, but was once just a Galka, one of the game's playable races, and my main character). The game builds the story off of your own memory, and in some ways on a kind of interactive real-time mimesis, as opposed to throwing one thing at you after another.

To bring in a connection from poetry, FFXI has a ton of whitespace, the differentiating element between poetry and other literary forms. There's space in poetry, the whitespace, for the reader to think, to interact and form his/her own judgments. A line speaks to the reader, the reader speaks back in his/her own mind in the empty spaces of the page, before the poem moves on to the next line. And I think FFXI has a similar flow, a kind of poetry in its openness and its expression.