To begin, it helps to understand that things get under my skin easily. I was running Word 2004 on a recent Intel Mac, and spent two hours toggling options and configuring my environment to try to get rid of a 50 ms lag in characters appearing on screen. (I never was able to, and ended up using Word 2010 on a PC to do that piece of work.) I argued with a guy for three hours about the use of a particular piece of iconography and the role of consistency in UI design, because a visual confusion was causing me thirty seconds of inconvenience per day.
So basically, I'm an asshole (to many of you) or particular ("OCD") about a couple things, which can be roughly boiled down to "responsiveness" and "flow". I'll upgrade my phone to reduce typing lag, I won't use a device that's too old if fast tasks aren't fast—app switching, typing, mouse input—and I hate chrome, animation, and interfaces that reduce application interaction speed. This leads into flow: if a button isn't clear after using it five times, or I mis-hit a menu because my understanding of an app is different from the designers' intent, I get increasingly frustrated. Adobe Reader using the Windows XP file dialog for saving/opening files (as opposed to the Windows 7 version) is one of my pet peeves. Thus, the main reason I prefer OS X is that there are so few exceptions: Apple's "update or get left behind" dictum to software developers is, in my mind, a virtue, because when everything is up-to-date everything behaves predictably and I can flow more easily.