Being able to push graphics through an ssh session through X forwarding
tunneling traffic over ssh tunnels
ability to target sound from one machine to another using PulseAudio
being able to write powerful scripts easily that can perform many mundane, tedious tasks
being able to have those scripts email/text/call/submit webforms/etc.
having a system that is extremely flexible and customizable to your preferences, hardware, computer needs, etc.
having a system that runs on just about any remotely modern computing platform (mid-90s onward)
having a litany of simple, powerful, flexible tools that perform a limited task set well
being able to chain these tools together to accomplish much more complex tasks
having a cornucopia of choices for secure, lightweight servers of all job descriptions (mail, web, source control, etc)
sticking it to Microsoft and Apple
I could go on, but that's just the shortlist that springs to mind