IPv6 packets can still be fragmented, but only at the source. IPv4 fragmentation has only worked this way in practice for a long time.
Private addressing is still needed with IPv6, it's a crucial part of how address allocation works, and it's the only way to reliably connect to a client-like IPv6 device on the local network, since its public IP address will change all the time for privacy reasons, assuming it respects best practices.
Routing is only simpler if the ISPs actually hand out the large prefixes they are supposed to. Not all of them do.
DHCP is still required for many use cases. So now you have two solutions for handing out addresses, and you need to figure out when to use SLAAC and when to use DHCP. This is strictly more complex than IPv4, not simpler. SLAAC is mostly just unnecessary cruft, a cute little simple path for limited use cases, but it can never replace DHCPv6 for all use cases (e.g. for subnets smaller than a /64, for communicating additional information like a local DNS server or NTP server, for complex network topologies, for server machines etc).
* First three points matter more on bad connections, but are less of a problem on good ones.
* Private addressing is a feature, not a bug, in the datacenter.
* NAT is a feature, not a bug, in the datacenter.
* Simpler routing matters more on bad connections, but is less of a problem on good ones.
* DHCP is a feature, not a bug, in the datacenter.
Overall, it adds features that I don't need in my datacenter, and takes away others that I do and now need to add back. Like I said: it's great outside the datacenter, not so great inside it.
> * No more private addressing (unless you're a glutton for punishment).
The question of whether or not you use private addressing is, AFAICT, independent of the protocol. I mean, there's no material difference between private and public addressing.
> * No more NAT (see above).
Ditto. You don't have to NAT over IPv4, and you can NAT over IPv6; and - you may want to or need to, depending on restrictions on your connection.
* You can't fragment packets.
* The redundant checksum header was removed.
* No more private addressing (unless you're a glutton for punishment).
* No more NAT (see above).
* Simpler routing.
* Doesn't require DHCP.
It benefits hugely from the lessons learned with IPv4.