Under IPv4, I often use nmap to scan my entire IP range to identify newly-connected devices and update my documentation, track down and shut off things that don't belong on the network, etc. I even have tools that do this automatically, for instance my AV software scans a defined IP range and then installs AV software on anything it can see in that range.
That's going to be infeasible under IPv6, as I will go from scanning a few thousand IP addresses to many quntillions.
What will the alternative be? Will routers/switches be able to report what IPv6 addresses they've seen lately so I can port scan everything on the network? That's the only approach I can see, but I expect that SF will have more and better ideas.