My list of impossibilities:
Spotmeta-like support in Spotlight
Working rsync -E
AES in Secure Transport Protocol (Security.framework)
Open-source XNU :)
OpenBSD's PF ported to OSX instead of ipfw2
isakmp instead of racoon
Better VPN support (not need cisco and apani's vpn clients?)
Uninstall (dependency aware) capabilities for [m]pkg (PackageMaker) -- or a real package mmgt framework like DarwinPorts.
ZFS (ya right) (and dtrace while you're at it)
... I'm sure there are a lot more, these were just pulled out of the air.
What Do You Want out of Leopard?