Windows Users: 99% of Windows users are stupid. Thats where we get the term user friendly. Makes it so that dumb people can run it. This is why there are so many viruses, Because stupid people are so easily exploited.
Mac Users: For People that are obsessed with the old "Its good for video editing" crap. It's not that great anymore. PC's can do the exact same thing, and beyond these days. There are so many advancements with PCs every day, but practically no one is advancing the Mac. It's not a profitable thing to do. Mac Users think theyre being smart by puting themselves above windows, but really, they limit themselves by restricting their application production to only a small few corporations.
Linux Users: The Elite; It takes a bit of know how to run a linux box. There are very few viruses written towards linux distributions. Plus, on the off chance you do get a virus, If your using a linux box. More than likely, You can get rid of it. Plus, for those of you saying that you can't do as much on a linux based pc as you can on windows. Well, Sorry Charlie, Incorrect. You can do EVERYTHING that a windows pc can do. (Thats right, Even Gaming, You just need to know where to get the right tools.)
I Feel That Windows will get the most competition from Linux in the near future. Even with some distributions branching off and charging, they are even still less expensive than windows, and you can install all linux operating systems on multiple boxes.
Also, I don't really know much about it, but We can't rule out Google's rumored operating system. I really think googles goal is to take over microsoft lol Cant wait.