The first two were havens for cheaters and exploiters. I should hope that makes them unique. Sadly, I don't think it does.
Norland... Well... They were unique. But I'm not a big fan of their style of "unique". For my money, RedSpan was a better style of unique. (Although I never played there myself.)
As for Arcachon, they're a plucky little realm. They definitely don't have any intentions of rolling over for anyone. And it seems like they're having a lot of fun instigating, and keeping OW and Arcaea on their toes.
I was thinking about joining Arcachon. My character Jupiter is on an island wide travel quest to the north right now to find a new realm.
Cathay was sadly too quiet for my taste.
Seems like they are a rather popular realm too, player wise. 35 nobles for that small realm with 1 duchy, compared to PoZ's 37 nobles for 5 duchies.