I'm happy enough with Leopard's features, but its aesthetics are a step backwards. "Aurora" is lame cover from a bad science fiction paperback, and the new icons are eyestrain drab.
Fussing under the hood with system icons is a nuisance, but for $29, CandyBar from Panic makes it painless. It provides an iPhoto-like way to manage your icon library, along with an interface for applying your choices system-wide.
http://www.panic.com/candybar/
It's particular nice to be able to try a new environment on for size, change your mind, and revert with a few clicks.
-Bryan