Considering I work in the scientific research field rather than the creative fields that seem common on Macrumors, I'll have to say POSIX-compliant UNIX at the command-line. No more using Windows for a mostly usable desktop GUI and a large application library and Linux for coding and good automation command-line tools (although Ubuntu Linux has begun bridging the desktop GUI issue). With OS X I can just use the desktop most of the time, but open an iTerm and build a quick C or even PERL program to manipulate some data file and set up a CRON job to run it when needed.
Second choice is that Macs are what we are given to use at work, so I wanted to have one at home, too. (Hey, if Windows users can use that excuse, so can I!)