Ease of use
Ecosystem (apps, drivers, etc.)
For background, I teach Linux for a living, so I am definitely more advanced than average. Still, installation was intuitive, fast, and painless. Not one piece of hardware in my Toshiba Satellite required additional drivers. Even my WiMax card was recognized. Interface is fast and incredibly customizable. I added my favorite window managers, Gnome 3 and Fluxbox, although I find myself almost exclusively using the native Unity interface. I find it more beautiful and elegantly designed than my MacBook, and that is saying something.