woah, easy on the hate, and easier on the bulls***t.
Mac OS X is POSIX compliant, and Linux flavors typically do not get certified (because of the costs).

Both are POSIX compliant, so what they discuss is th 5% difference.
You use just the same default command shell /bin/bash
Both (all) use a GUI based ob X/11 window manager - except Android that has its own.
All POSIX commands are available - 100%
Just as Maemo has parts of its code proprietary to Nokia, so has OSX parts of their code proprietry to Apple, and those that write 3-letter acronyms about which parts of the code, usually does not have a clue about this. I do not like the Wikipedia entry about Unix/Linux as being monolithic, this is not related to threading, but to the code hanging together from top to bottom. It certainly supports multiple threads run through it, every task has its own threads to the file system. Sorry friends, but I have coded some of this. So drop the acronyms.

The Flashing scripts for Linux runs without modifications on Mac OS / OSX.

Much of the applications on Maemo that is coded in Perl or Pyton - are being interpreted and wil run with tiny modifications on a Mac - and you can take complex scripts e.g. for testing the file system from the Mac to Maemo - and bluntly also on Android as long as they are not using X11.
There are some huge differences, like square corners on the windows, buttons being round and with/without shadow that makes the difference. These are the kind of differences we easily can remedy by changing theme.
We have a similar protection system, not something that has been added on the top, but imposed from the bottom, like you cannot execute a file that a picture has downloaded in your browser. You execute everything in a shell, and we can impose safety constraints, such as that the scripts originates from a browser, is owned by a Internet resource - on Android: It was stored on the SD card before executing.

If you consider round corner versus square ones are important, study art and return with a cute theme.

Mac OS does not use X11, but it can run it. Xquartz (the mac implementation of X is not even preinstalled anymore on mountain lion)

GIMP runs natively on a mac while inkscape needs X11. There's a noticeable difference in the behavior and thus the usability of those two programs because of that.

Mostly an accurate response...but unix is to an operating system! darn kids! get off of my lawn! Unix was an operating system long before Linux, DOS and the current mac os existed. Before you were born, we all used variants of unix. BSD is just one flavor. It's always been an operating system, free and open source even though developed at Bell labs...just other companies developed their own and marketed them...like Sun etc.