What I miss in OSX 10.5

The ability to have a different input system per window

This setting was present in Tiger under "International > Input Menu" at the bottom of the window, the same location in Leopard only indicates the keyboard shortcut to change of input method.

Use case: you translate from a language that requests a different input system from the language you translate to. When you need to type a word in a dictionary to check its meaning, you change of window, change of input system, type, get the results, go back to the original window, go back to the original input system and type. Having the OS remember which input systems goes with which application is quite a time an annoyance saver...

Clear visual clues to identify folders in the Dock

John Siracusa has a full page about that on Ars Technica, with plenty of nice screenshots:

Mac OS X 10.5 Leopard: the Ars Technica review: The Dock By John Siracusa | Published: October 28, 2007 - 11:36PM CT