What impacts does data center and server have on party-play/gameplay?

[Raldo]

Server - Affects who you'll see in the open world. You can visit other servers on the same data center though, so your server isn't terribly important**
**However, you can only use your retainers while on your home server.
**You can only own a house on your home server.
**Are free-companies cross-server yet? Not sure.

Data Center - This is the most important choice. You cannot interact with players on a different data center. All duty-stuff is data-center-wide. The Duty Finder pulls people from any server in your data center. The Party Finder shows parties from any server in your data center. So long as two people are on the same data center, they can play together (and with world to world visit, can even play in the open-world together).