The IPv6 spec was being modified up through 2017. It has more kinds of addresses that behave in fancier ways, with one host having multiple. The very first thing you see with ipv6 is your nice memorable ipv4 addr replaced with a long hex string with some ::s thrown in. Local DNS is commonly recommended with ipv6 for that reason, which maybe is just some misguided advice because it sounds crazy. I guess you could assign and memorize ULAs?

NAT is technically complicated if you're looking inside it, but most people aren't, and for them it's really easier to think about. You've got a public and a private, and there's a very strong default that private isn't exposed. People screw up firewall rules all the time or routers have bad defaults, but it takes more deliberate action to publicly expose a port over NAT. Plus you don't need privacy addresses that way (introduced to ipv6 in 2007). I know "NAT isn't security" but for most people, it is.

Still not even sure what the accepted default firewall behavior is in ipv6, cause some people say "ipv6 lets any device do p2p by its own choice" and then when you ask about security, "your router firewall should always default-deny anyway," so which one is it?

> The presence of the IPv4 stack isn't what blocks the adoption of IPv6

It is. Like they say, most technical problems are really people problems, especially this one.