This is a good overview. I think the difficulty with IPv6 is that people rely on all of the crutches invented for IPv4 as features: private addressing NATing gives you security (it doesn't) and portability (it does), IPv6 usually uses subnets per physical location making failover difficult, where as IPv4 will use bgp announcements to failover public IPs, etc. I'm not saying one way is better than the other, just that IPv6 is pretty different and people very much have a IPv4 world view.