>What if I don't like the new look of Windows or OS X? I can choose not to upgrade the entire platform, but that's it, unlike in Linux land where I can upgrade piecemeal.
You underestimate the customisability of Windows. Its ecosystem is mostly proprietary and closed-source, but that didn't stop people from modding what they could - just look at all the "post your desktop" threads that appear in various forums and you'll see lots of examples that are barely recognisable as Windows anymore. A lot of these involve little more than copying/replacing a few files, or some registry editing, so it's actually not so difficult at all.
In fact I'd say that "upgrade piecemeal" with Linux is made more complex because of all the dependencies that often arise with various apps all wanting very specific versions of libraries, multiple competing standards that do not interoperate (count all the GUI toolkits...), and emphasis on portability that tends to reduce the availability of binaries.
Some people go as far as replacing the shell:
http://en.wikipedia.org/wiki/List_of_alternative_shells_for_...
...or even removing/replacing more fundamental components:
http://en.wikipedia.org/wiki/Software_remastering#Microsoft_...
As for OS X... I agree there's not much customisation going on there, but I'd attribute that more to the demographic of Apple users. If Apple had the same marketshare with Mac OS as Microsoft has with Windows, I bet there would be a ton more modding happening.
I've used both Windows and Linux, and can say that I like and hate both, but for different reasons.