I have played on a JP server and it is exactly like the NA servers except less crowded because of the time difference. And quieter, regardless. Even in the beta when I played on a JP server and there were loads of players everywhere, there was not as much chatter in /shout as I was used to seeing on a NA server. Was like that in 1.0 too though when we were all on JP servers.

Aside from that, and the possible language issue with your fellow players, the other thing to consider is your ping. That depends on where you live though. As an example, I live on the west coast of the US and I ran a ping test which showed only a ~20 ms difference between the NA and JP servers. But if you are on the east coast near the servers it may be a bigger deal.

As for transfers, the example given on the page for the world transfer service (coming mid-Sept) used a JP/NA transfer as an example, so I imagine they will allow transfers between the regions.