Linux. Why?
No, the categories of answer I'm not looking for are "A superior OS to anything else", "Free" or "It's Not Windows". Those are all givens.
My answer; hacker ethic. I mean hacker in the Steven Levy sense of the word, not the malicious genus of hacker. The early adopters, the innovators, the people who delight in the obscure art of doing something nobody else is doing. Today's Linux brigade is the same bunch of people that pioneered the age of personal or home computing in the seventies and eighties. Bill Gates was a hacker, probably still is deep down underneath all that cash. A love of Linux today is very similar to the love of making LED's flash in strange sequences just because they're attached to a home-built computer controlled daughterboard attached to your Apple II's I/O port, circa 1982.
The problem with the hackers that grew up to become the 1990's PC enthusiasts that pioneered today's black, flat LCD home appliances is that, slowly, their 'thing' became everyone else's thing too. PC superstores opened up, prices fell, complexity disappeared and user numbers grew. The bar was lowered, all you needed to join was ignorance and cash, the modern day replacements for a hacker's love of learning something the hard way.
I visited a computer fair yesterday. It was all refillable inkjet cartridges, second hand hard disks, various components, cases, psu's. In other words everything you needed to buildassemble your own PC. Nothing exciting, nothing new, no great invention or home made projects. Just a faint whiff of cheap'n'cheerful with a liberal sprinkling of pirated software, three-versions-of-out-of-date .
You used to have to read pages of a manual two or three times to learn how to format your computer's hard disk. And I once bought a ROM based word processor in 1984 that required me to open up my computer, root around among cables and scary circuitry, find an empty and compatible socket and plug it in, the right way round - very important - without breaking or bending the 64 tiny little metal pins. Today its click, bang and you're there in a nano-second. This fact does not amuse hackers. Too easy. If everyone else can do it then where's the fun anymore?
Hence Linux. Just to be different, just to isolate themselves from the mainstream once again. I installed a Linux MSN Messenger client the other day, I had to unpack, arrange the portions of source code in the correct order, learn how to compile it and then assemble the whole shebang into something I could actually create a shortcut to and run. Having said that, I did feel like I'd actually achieved something after the 15 or so minutes to took me to navigate each step of that process, more than once I might add.
Unnecessary complexity, now you're talking. Back to a place where if you have to ask then you don't belong.