Thinking outside of IP, IPv6's design is of high maintenance. The design was based on 1990's technology and is much worse than IPv4. There's no Identification field, which makes it very hard to debug. It uses this linked TLV headers instead of stating the total length in the beginning, and that makes the implementation very hard.
Ipv6's design is just so bad, and I don't understand why people won't start to work on the replacement.