Bad stuff (need to be cautious)

IPv4 mapped address
Twists in new API (getnameinfo)
Heavy use of tunnelling
No NAT -> firewalling/fire suit is important
No NAT -> more p2p apps -> difficult to filter/police
Extension header chain -> difficult to filter
Multicast -> difficult to track down
Additional Complexity due to multiple protocol families
Specification unclarity/implementation incompatibility