One major problem is dual stack. It doubles the workload for very limited benefit. You have all the downsides of making ipv4 work in the first place. You've then got all sorts of messes like NAT66 (ipv6 was supposed to get rid of NAT), a lack of clarity on which patch to use (NPTv6 and NAT66 are two different options for the same problem, a problem which was built into ipv6 in the first place), messy hacks like DNS64
Instead had the approach been ipv6 only from the start, with no dual-stack, having the OS transparently deal with sockets to ipv4 devices by converting to the ipv6 mapped address (:ffff:xxxxxxx), and thus eliminating the need for dual stack from the start, things would have moved far far faster. You'd be able to communicate with ipv6 by using stateful NAT at the edge of your ipv6 network (as you do now at the edge of your RFC1918 network), you could expose services on your ipv6 only devices with natting (as you do now).
You'd still have A and AAAA records, your client having an ipv6 stack could prefer AAAA instead of A, but if it needed to use an A record (or someone just tried to connect to 12.34.56.78) the stack would have gone "ok I'm ipv6 only, I'll connect to :ffff:12.34.56.78" and rely on the network to make it happen.
Throw in things like NPTv6 and 464XLAT from the start (rather than 16+ years in) -- the addons which were created to address the fundamental architectural flaws in ipv6 -- and you'd have had a far smoother transition.
