Sufficient to have more than one subnet. In IPv6 you can't easily have a subnet that's smaller than /64, so even though /64 = 2^64 addresses, it's still just a single subnet. In IPv6, we need to start thinking in terms of number of subnets rather than number of IP addresses.

Why is having one subnet insufficient? I use multiple subnets (I have a guest network), and I don't want to have to use NAT with IPv6 when my ISP has effectively infinite address space. Of course, I'm not an average user, but remember that 10 years ago only power users had home routers, and for everyone else 1 IP address was enough. Now everyone has a router. There are already some consumer access points that tout a "guest network" as a selling point, and there may be other consumer uses for multiple subnets in the future.

a /64 gives you a single subnet with effectively infinite size. A shorter prefix would allow you to have multiple subnets for isolating things like gaming devices, guest networks, VPNs, etc. For IPv4 the ability to do this was basically a pipe dream, for IPv6 the addresses are there, we just need to coordinate how ISPs hand them out and how consumer routers manage them.

In short, one /64 is sufficient for duplicating an IPv4 like situation. A /60 or /56 gives us more room to innovate in new types of home networks.

yeah, a /64 is more than 2^32 times more than all of current internet... i don't think the GP really meant 'sufficient', or i'm missing something.