The 10.5 Dock, as seen at the bottom of the screen, has a 3D effect. If you toss the dock to the side of the screen, however, you get a nice 2D alternative Dock. If you'd like the 2D dock at all times (that is, even at the bottom of the screen), open Terminal and type the following command:
$ defaults write com.apple.dock no-glass -boolean YES; killall Dock
The Dock will then restart, and you should be seeing the 2D dock at the bottom of the screen, like this (click for a larger version):
This one simple change had a dramatic impact on my feelings for 10.5 -- I really, really dislike the 3D dock. This 2D version isn't perfect, but it's a marked improvement over the stock version. BTW, if you like the dimmed "hidden app" indicators in my dock shot above, those are from this 2003 hint
that still works in 10.5.
If you can't decide between the two docks, and want to make it easier to flip between them, my coworker Dan Frakes whipped up a couple of very
simple AppleScripts [34KB download
] to make that task easier. You could use Butler or a similar app to put each script on a hot key, making it even simpler to toggle. No toggling here, though, as the 3D dock has seen the last of its days on my machines.]