I don’t mind the 3D look of the OS X 10.5 dock, but I often prefer the 2D look of the old dock. Plus, I prefer the more-obvious running app indicators from the 2D dock; I find these hard to see in the 3D version. Also, sometimes I just prefer the same look for my dock whether it’s right- or bottom-oriented. Also, hidden apps (⌘H will hide, not minimize, an app’s windows) can be configured to show as transparent icons.
From a terminal:
defaults write com.apple.dock no-glass -boolean YES
defaults write com.apple.dock showhidden -boolean YES
Credit should be given to macosxhints.com for this.