1. The small memory requirement, VMWare recommends 512MB for ubuntu and 2GB for Windows7.
2. sudo get apt install, if need something run it, stuff installs and boom you're up and running.
3. ruby on rails, like crack for coders, quickly prototype active record apps, especially with the scaffolding stuff.
4. The OS makes you embrace the shell.
5. It's just cooler to say "I'm using Git on Ubuntu" than I'm using "Team Foundation Server"
6. The openness of everything, want to know how something works, here's the source.
7. Connected user community with great support from the irc channels.
8. The file system organization.
9. The cool t-shirts.
10. The feeling of superiority you hold over .net developers.