To me too, OS X is very important. The way it works and everything else that is not objective and deals with "taste". On the objective side of things, I like the ability to be in *nix environment, run proprietary software (such as Adobe CS, a reasonable Skype, iTunes, MS Office or iWork - I definitely don't like OOo or forks, etc.) amongst other things. On the middle ground of objective and subjective, I like the fact that my OS doesn't keep slowing down with time - this can be measured but I'm not aware if someone has.
I'm often surprised that the value of OSX and its built in conveniences is not factored into a comparison. Preview, built in file extraction, TimeMachine (which does not really have any comparable alternative on windows IMO), Calendar, Mail etc.
Some of these apps are built into windows (but not with the same modern feature set) and most are available for free, but OSX seems to have hit a good selection of apps that you can use out of the box.
Edit: Also that I upgraded/time machined from my 2008 white MacBook to my current generation MBP on Lion without loosing any performance, apps or data was a remarkable time saver.
Agreed: The best thing about OSX is that it combines a great graphical interface and support for commercial software with a Unix command line.
You can achieve something similar with Windows 7 and a Linux VM and some file system mirroring tool (VMWare sharing or Dropbox or something). But nothing beats OSX for slickness.
