I'd played a lot of MMOs prior, but XI was the first that gave me that 1st week sense of awe and wonder... and kept a majority of those feelings for years. Eventually the Dunes and other places got old, but every now and then you stop and take a different path and just get amazed by the attention to detail... something that was very lacking in a lot of the much more "shiny" MMOs.
Any way as mentioned, experiancing almost every aspect of the game with a single character through job switching and subs was fantastic.
The story, something that I don't think will be rivaled until the BioWare MMO and XIV come out, was fantastic... even when I had no clue what was going on, it still made things feel epic.
The mixed community, something that was amazing when we first got into the game and then became even more diverse with the EU luanches. I really like being able to group with people I can't understand and be able to accomplish things. The only other MMO that gave this to me was EVE.
FFXI:Sylph - Perrin 75 Hume THF; Retired (At least from my use any way)
EVE Online:ScraperX; Retired
WAR:IronClaw- Peryn SW;SkullThrone- Grymloc BO; Retired