One thing that isn't discussed very often but is IMHO one of the bigger roadblocks for IPv6 deployment is the Berkley Socket Interface. The API that loads of code is built upon to do internet communication.
The problem is that the API is too low level, or more specifically there is no high level API for it. Ideally it would have a function that looks like:
int sockfd = connect_to(hostname, port, SOCK_STREAM, options_bitfield);
This would allow the stack to work out the details on its own and automatically use IPv6 if available. Or whatever future protocols can provide you a STREAM socket. The old interface could also be available for people who need to do low level stuff, but most of the time this would be sufficient.
The problem is that the API is too low level, or more specifically there is no high level API for it. Ideally it would have a function that looks like:
This would allow the stack to work out the details on its own and automatically use IPv6 if available. Or whatever future protocols can provide you a STREAM socket. The old interface could also be available for people who need to do low level stuff, but most of the time this would be sufficient.