[tardedmeme]

You just send packets to the other party's address and they send packets back to yours. Both parties know their address and you don't need a relay in the middle.

[ekr____]

This really isn't the case, because people still have firewalls.

[pocksuppet]

It's not really relevant in this case since one endpoint is a massive server farm.

[hnav]

It is because most of their complexity is in routing packets. With IPv6 you can just have the thing handling the conversation directly addressable by the client. The last 64 bits of a v6 let you have billions of instances in a region.