Iljitsch van Beijnum in "There is no Plan B: why the IPv4-to-IPv6 transition will be ugly" on ars technica:

"While IPv4 and IPv6 are very much alike to users and applications; 'on the wire', the protocols are completely separate and don't interact. In routing, we call this the "ships in the night" approach. (Hopefully, at least one of them has radar.) The advantage of this design is that there is no need to change existing IPv4 infrastructure——IPv6 is simply added as a new protocol. All the limitations and mistakes that are part of IPv4 are left behind. The problem with this approach is that the first person who wants to turn off IPv4 has to wait for the last person to add IPv6."

Running out of IPv4 address space, no ubiquitous DHCPv6 support, no IPv6 NAT, too much IPv4 NAT, the threat of DNS whitelisting, and no Plan B, oh my!