I recommend the lecture of this: https://www.nanog.org/meetings/nanog46/presentations/Sunday/... (deployment document of ipv6) then proceed to the glitches section.
I recommend reading nanog mailing list for having a glimpse on the added complexity of dealing with IPv6 in real life.
Just peak random mails, it pops up once a month. Like for instance provider turning to LS (Large Scale) NATing or Carrier Grade NATING rather than going IPv6 for rationnale reason: IPv6 engineers are hard to find and LSNAT (+dual stack eventually) since China has proven valid has both a way to handle the scarcity of IPv4 addresses and the need to control user's traffic.
http://mailman.nanog.org/pipermail/nanog/2014-July/068892.ht...
IPv6 is not bad, it is kind of bloated and requires a tinge more training than IPv4. Plus there are the concerns about privacy (but some IPV6 guru will come up with ULA or another obscure trick. It is IPv6 realm: everything is clear on the paper but is seems confusing to quite a few real life engineers since it requires a lot of RFC stacking in the brain)