I'm just repackaging what others have said, less eloquently, but her goes...
Privacy
There's no paranoid itch in the back of my head. In an increasingly Orwellian world, GNU/Linux is a shining beacon of hope.
Performance
Older machines, of which I have many, run better than most modern Windows computers. And, my modern workstation is an absolute beast on Linux. Windows feels like driving a school bus, where Linux is like a tuned street bike.
Power
I can simply do more with Linux, it doesn't come with training wheels and multiple walls to prevent me from doing anything useful; it begs to be useful.
Community
The idea of FOSS, and what it stands for, sits well with me. I have great respect for those who sacrifice time and effort to make Linux what it is today, and though not everyone is wonderful, the overwhelming majority of users are friendly and helpful. With Linux, I'm a not just a consumer, I'm taking part in something.
Customization
My desktop looks and behaves exactly how I want it to. There's no bloatware, no bogus junk that I'll never use, just my own stuff. With a few key strokes, I can set up the same distro on a simple netbook, or on a headless web server.
Reliability Back in the Windows 95/98 days, I remember feeling lucky if I made it six months without having to reinstall. During that same time, my Linux machines didn't even need rebooted. To be fair, Windows has gotten much better, but so has Linux.
Perception
Nobody knows what the hell I'm doing on Linux, and they automatically think I'm some sort of guru (I'm not). Though it's based on misconceptions, Linux intimidates people, and I'm completely fine with that. It works out as instant proof that I know how to use a computer. Countless projects have been thrown my way simply because I'm the guy who uses Linux.
Other
Kind of pie-in-the-sky, but there's this thing that's tough to pinpoint. Back in the day, when the Internet was young, there was a feeling of being on the forefront of something great. Linux gives me that same feeling today. My idealist mind tell me that when IT comes out of its infancy, and the Apples and Microsofts of the world are long gone, Linux (or some form thereof) will be stronger than ever, and that I'm on the right team.