Summary

Should make the code address family indepenedent, as much as possible.
Should avoid writing AF_INET nor AF_INET6 directly.
stick to getaddrinfo() and getnameinfo()
The Advanced API: raw IPv6 sockets, extension headers.
note differences from raw IPv4 sockets.
compatibility/availability issues.
IPv6 multicasting
use standard API defined in the basic API spec.
Routing
unicast: basically trivial, note KAME specific hack of link-local addresses.
multicast: API standardization issue.
Kernel Programming
be more careful about validation.
debugging: remote debugger and statistics are useful.