* privacy addresses are great

* deriving additional addresses for specific functions is great (e.g. XLAT464/CLAT)

* you don't get collisions when you lose your DHCP lease database

* as Brian says, DHCP wasn't quite there yet when IPv6 was designed

* ability to proactively change things by sending different RAs (e.g. router or prefix failover, though these don't work as well as one would hope)

* ability to encode mnemonic information into those 64 bits (when configuring addresses statically)

* optimization for the routing layers in assuming prefixes mostly won't be longer than /64

… and probably 20 others that don't come to mind immediately. I didn't even spend seconds thinking about the ones I listed here.