Pasting over from the other thread:
'My main concern with any cross-realms instancing is that it really dilutes the communities of individual servers by putting them in to a sort've global pseudo-community (anyone who played WoW can attest to this). The same effect will probably happen if implemented to FFXIV, though most likely due to a lesser effect since it has no PvP or end-game 'raiding' [at current] (which were pretty much the staples of in-game communal growth for WoW, from my PoV... cross-realming for PvP anonymized a close-knit, competitive community, and cross-realming for dungeons did certainly encourage ninjas and morons, though I don't think this game will have a huge problem with that given the general community).
The one perk is that it makes things more accessible for dying servers by pooling all the populations into a cross-realms instance queue. Hopefully it doesn't come to that months down the road, after subscriptions are implemented
Edit - We should really wait and see on this one I think. At current, almost every "party" I've seen in the search queue has been someone advertising for an LS in Japanese... but if the population issues are ameliorated after a strong PS3 launch, they won't have to implement cross-realms features because of this sort of thing. Also, I think this game is in desperate need of an in-game LS advertisement channel =3 '
TL;DR - L2read
