I still never understood why they couldn't just extend IPv4 by adding another octet, and then making all existing IPv4 addresses 0.{current_address}. This would give us about 1 trillion addresses and would make things dead simple.
The only glitch that we ran into in the last nine years was a (self created) configuration error, where we statically added default gateways on hosts/servers that pointed to the router. Those Router IP addresses had been auto-generated VLAN MAC addresses, which where then concatenated with the IPv6 network /64.
Unfortunately, the Cisco routers we were using, generated those EUI-64 addresses for the vlans based on the order they were created - and, after a power cycle, they were created in the order they appeared in the configuration - and all of a sudden our servers no longer had valid routes.
Lesson learned - don't use static gateways on an IPv6 network, let your routers advertise themselves, and, if for some reason (possibly a bad one), you really, really want a static gateway , make sure you don't rely on the auto-EUI-64 generation ability of your router.
Other than that - the protocol has been an easy drop-in replacement for IPv4.
The "Why" of not just adding 0. to IPv4 - is that IPv6 does much more than just increase address length. First of all, it eliminates CIDR - which is the source of many network misconfigurations. All networks in IPv6 are /64s. Further to that, you get stateless address autoconfiguration - nodes can now determine their own address without having to rely on a stateful registry somewhere. Just concatenate the network+mac address (or random number for the privacy sensitive), and you have a routable IPv6 address.
Broadcast is eliminated - and solicited nodes multicast is a very elegant replacement for services like neighbor discovery (rough equivalent of IPv4 ARP)
I could go on - but, I don't know very many IPv6 network operators who dislike the protocol.