Sadly, I'm sure that Orsinium/Wrothgar will be like Craglorn, where each faction gets their own instance. But it would be nice if there was just one area of the game where members of opposing factions could meet in a non-PvP environment and socialize, run dungeons/grind together, etc. I've got friends in other factions who I've never even seen in-game before. It makes it all-but-impossible to have a true cross-faction guilds too.
It's not like I can just create a level three character fresh out of Coldharbor and go run vet dungeons with my friends and guildees from other factions. And I'm sure not about to level up two more characters in the other factions to vet14 again. One was hard enough.
If you want some excuse for this, just say that, in the epilogue to the plane-meld unpleasantness, Meridia decreed that Wrothgar would be the one region where the three factions could meet safely--with her assurance that none could harm the other. There you go, lore-friendly and everything. Now let's all go drink mead together and do some dungeon runs!