As a longtime XI player, I agree that the game had a sense of community that felt a lot more 'real' than pretty much any MMO I've ever played. And that is largely because every significant piece of content in the game had an absurd amount of preparation time, overhead, and organization required before anyone was successful at it.
Actions had weight. Neither the game nor the devs held your hand. Until the game's much later stages of life, if you wanted to get anything done you were committing a significant amount of your time and attention to it along with other people, which meant that everyone took the events happening in the game much more seriously.
The final product was immersive(and time-devouring) in a way you'll only see fictional MMOs describe, and I doubt we'll be seeing that kind of model again for a long time, if ever.
And just for the record, WoW barely touches on that realm. 40-man raids had some of it, because the upper-level organization had to be a different kind of serious for them to succeed. The earlier forms of open-world PvP/Battlegrounds had it if you were a Honor-monger, etc. But that was just a specific strata of player, whereas in XI--the game where it literally can take an hour to do simple productive tasks--everyone was affected by it at all levels of play.