I think anything different is fascinating for most people. Possibly not as much for Mac and Linux users though. Both account for a small percentage of the computer market and tend to be a lot more loyal to their platforms because of this.
When I was only a PC user, Linux was my main fascination.
I loved Linux because it was different than Windows. I especially loved running the WindowMaker window manager which is based off of the NeXT GUI.
There was a time, about 4 years actually, that I was anti-MS. I ran Linux exclusively at home. It was great and worked well for me. I never considered a Mac then because 1, they were rather expensive and 2, I couldn't stand OS 9.
I gave MS another chance with Windows 2000 Advanced Server. Honestly, it's a great OS.
I knew when OS X came out I would buy a Mac. I loved NeXTStep so I knew I would love OS X. My PC is being neglected more and more each day thanks to my PB G4 and OS X.
For what I use OS X and 2000 for, neither one is better than the other. I love the elegance, productivity and stability of OS X and the speed, features and stability of 2000 Advanced Server.
I didn't like XP but I hold out hope for Longhorne.
I am most excited about Jaguar right now and await my copy in the mail.