Most of the issues are not debian specific. But did you every try to use nrpe (from nagios) to check an ipv6-only host? Or use the httplib python library to an ipv6-only host with an https-connect? Just to name two examples of the interessting journey with ipv6-only machines. We are just not there. "We" as the linux world at large, not only "we" as in Debian.