Keyboard customization tool
As for me the ability to freely customize keyboard keys is a deal breaker. Right now I'm pretty happy with OS X and KeyRemap4MacBook. The latter allows me to use a lot of finger saving things, such as: switch between keyboard layouts with separate single keystrokes (right shift and left shift), caps lock + "some keys" translates to ctrl + "some keys", single press of caps lock translates to escape, using of vi-like hjkl for cursor navigation while pressing right command, etc., etc. (a lot of predefined settings http://pqrs.org/macosx/keyremap4macbook/list.html.en)
The nearest alternative to it on Linux is Gizmo Daemon (http://gizmod.sourceforge.net/). It doesn't have fancy UI, only Python scripts, but it gets work done.