Guild Wars 2

I see how it could be considered a downer, but when guesting is introduced this problem will be resolved. Then you will be able to do all things with your friends on other servers except WvW, this you can only do with people in your homeworld.
You could try to join your friend on an overflow server, not sure if this is possible though.

This is not possible, all characters will be on the same world. In the future guesting will be introduced, allowing you to play on other servers with people that you know.