One way would be that it would help identify individuals better since giant NATs will not be as popular. For example, I believe some IBM campuses are often all behind a single IP address.
as nat was only a method of dealing with lack of address space, and not a security measure, hopefully nat will become very rare...
Not really; most OSes now enable ipv6 privacy extensions by default, which means a client will connect with a different IP every hour or so.
At least some of the software designed to detect malicious behavior assumes the ip is in ipv4 format. Example: I'm pretty sure fail2ban doesn't support ipv6, yet.