Many computer games or P2P systems work around the pain that is NAT. But with IPv6 you just... remove the workaround. That's always going to be better.
If you like the "game requests a firewall exception by making a uPnP call to the router" model, there's no reason at all you can't do exactly the same thing under IPv6. The only difference is that games that don't have all the hacky workarounds will also work.
That they are widely deployed, reasonably predictable and quite transparent to the user. Which means that in the default configuration a user has a secure setup that apps however can partially circumvent to enable incoming traffic on UDP ports. Many computer games or P2P systems exploit this setup quite successfully to set up P2P networks without any user interaction.