IMO, the main issue with IPv6 is it's overall complexity compared to IPv4.
And what complexity would that be, if I dare challenge you? I say it's the contrary: IPv6 is dead simple. IPv4, having to deal with NATs and other networking hacks like it makes everything which should be simple complex.
Really: IPv6 is a no-brainer, and the only thing remotely "complex" about it would be that addresses are larger, and that would be because IPv6 was designed to accommodate a larger address-space. But since end-users will be using DNS anyway, that's not going to bother anyone except techies.
Techies who should be able read a few simple FAQs on the subject and be done.
TLDR: If you find IPv6 "complex", that's because you've been too lazy to look into it. I recommend you go do something about that.
Since you called me lazy (thanks for that), I'll respond. Keep in mind, I'm not remotely an expert nor do I claim to be.
A few FAQs? I think you're being a bit disingenuous. While the protocol itself is relatively simple to understand, the complexity related to its implementation is immense which is why we're still talking about it and not fully doing it.
Why it's complex:
1. The recommended approach is dual-stack. This means that network admins/system administrators will essentially have to manage two separate networks. Of course, this is assuming you have core-to-edge IPv6 support. If not, then you're upgrading all of your hardware.
2. Cost. Predominantly why we're still here. Often easy for us to push aside in our cosy developed world, but ignores an enormous number of legacy hardware deployed all over the world that will simply not work and, quite frankly, the solutions like tunnelling read a lot like the supposed pitfalls of IPv4.
3. Training. It's not a matter of a few FAQ's to learn, for a lot of technicians it is a matter of re-certification as all hardware has to be replaced which likely means new operating systems and firmware, proxying/tunnelling servers/devices ala Teredo/ISATAP.
For anyone that says it's a "no-brainer", I say that you have little to no experience in managing large heterogenous networks in cost-strapped organizations which, up until now, have had little to no reason to upgrade as the IPv4 address exhaustion issue does not really affect them (yet).
While you bring up quite a few valid points, I strongly oppose that "the complexity related to its implementation is immense". These so called complexities has to be weighed against the cost of keeping patching IPv4 networks into still working as functional networks. And that job gets more complex and costlier for every passing day.
With IPv6, most issues you have in IPv4 space would be the solved the instant you enable IPv6. All the hacks goes away. It's a nice, clean network again.
The cost of getting IPv6 capable routers should be negligible as anything worth the money bought the last half decade already supports it. Chances are if you haven't replaced it yet, you will anyway, due to stupendously increased bandwidth requirements we've seen the last years.
I'm not saying it's a free ride, but I'm saying it's a no-brainer. Because keeping patching the rugged IPv4 landscape we have now is not going to stay viable. And then you might as well invest in the proper solution right away instead of wasting money, time and resources on a stop-gap solution.
And what complexity would that be, if I dare challenge you? I say it's the contrary: IPv6 is dead simple. IPv4, having to deal with NATs and other networking hacks like it makes everything which should be simple complex.
Really: IPv6 is a no-brainer, and the only thing remotely "complex" about it would be that addresses are larger, and that would be because IPv6 was designed to accommodate a larger address-space. But since end-users will be using DNS anyway, that's not going to bother anyone except techies.
Techies who should be able read a few simple FAQs on the subject and be done.
TLDR: If you find IPv6 "complex", that's because you've been too lazy to look into it. I recommend you go do something about that.