my sincerest apologies if this has already been addressed (but I waded thru to P5 and do not have time to read the inevitable bitch-fest):
In a previous thread, it was debated that even though the dungeons will be instanced, the server would not be able to handle an infinite amount of groups going in at once. This would then (in theory) cause a lock-out on people waiting to enter the instance. As far as I know, they haven't yet confirmed nor denied if this would be the case.
Does anyone know what the final word is on that yet? It would kinda suck if the new content was bottle-necked like the "new" behests are.
*edit: also, I didn't really play XI so if you reference it I won't know what you're talking about.