1. It's free...legally free.
2. Almost every useful application you would ever want to run on Linux is free.
3. When you want to update to the latest and greatest version, with all the latest and greatest features, you don't need to buy a new version...and if you want to stick with the old, you can.
4. Security. Defaults are much better than Windows. There are so many free options for increased security, SeLinux and Grsecurity just to name a couple.
5. All you need to learn about it is out there somewhere, for free, whether it's online, in a man page, or in a forum or newsgroup somewhere.
6. Performance...Linux almost never forces you to upgrade your hardware, and it always runs better on older hardware than windows.
7. Simplicity. Yes simplicity. Most young people think Windows is easy because that's what they grew up with and had the most exposure to. That's like someone in the US saying english is the simplest language.
8. The choices...the free choices, are endless. What filesystem do you want to use, what do you want installed or not installed on your system, what graphical interface do you want to use and how do you want to configure it, which pdf viewer do you want to use, etc. Windows will never be that flexible.