Eh, Windows is fine, if the following apply to you:
You're a GUI-oriented user
You care about how pretty your stuff looks
You play many games on your computer
None of the above apply to me. I stopped using GUI-based menus and configurations when I learned how much time I was wasting ( I am a faster typer than using a mouse ) and I care for functionality and speed rather than looks. Finally, I simply don't play many Windows exclusive games. I do play the Elder Scrolls on my Windows installation, as well as PSO Blue Burst and Touhou. Those simply aren't enough for me to run Windows on a daily basis.
In addition I found Windows impossible to program for compared to UNIX, as the latter has a set of well-defined extensions to C that I can count on being part of the OS and not changing with each ABI/API release. This is a problem that Windows has, as Touhou 6, for example, won't run on the latest version of Windows, Windows 8, without a compatibility mode change to Windows XP SP3. While I'm familiar enough with how to do this, many Windows users don't even know what most of the definitions of the properties menus are for. On BSD, I can do many of the same things the properties menu does with a simple command change, rather than navigate, select, apply, ok. Example: Permissions on BSD:
% chmod 755 foo.tar.bz
Windows: Right click on foo, Click Properties, Security, Select Permissions, Apply, OK.
There was someone who called FreeNAS Debian based, that is incorrect, it is 100% FreeBSD and is funded by iXSystems, also the major fund source for PC-BSD, one of the most advanced desktop-oriented operating systems currently around. In fact, when one of the developers for FreeNAS wanted to make it Debian-based, he was kicked out of the project because his reasoning behind it was flawed.
OS X is a piece of work - it is Mach trying to masquerade as UNIX with the help of a few 4.4BSD-derived kernel enhancements and a brain damaged fork of the FreeBSD userland, with a custom GUI layered over the same thing. Problem is, there are significantly more compatibility issues with Mac OS between versions than even Windows: Mac OS Classic applications won't run since 10.4, PowerPC OS X applications won't run since 10.6. That isn't a problem for me, as I admire their willingness to break compatibility, as it is a waste of libraries, code and processor die-space for the latest version of 32-bit Windows to be able to run basic 16-bit DOS applications. I digress, because I've already harped on why I don't use OS X.
Finally I see Fats referring to the GNU/Linux as 'Linux' but this is incorrect, as Linux is just the kernel, and there are versions such as Alpine Linux which use a minimum of GNU software such as uClibc, etc. I use GNU/Linux to denote the differences between those, and non-GNU variants like Android and Alpine. I use Android, but I didn't really include it here as using a mobile based OS for a main OS seems pretty impractical. Again, as a Randian believer in philosophy, I disagree with the approach of the FSF/GPL/GNU project to licencing, as it stifles innovation and encourages monopoly and domination. Red Hat dominates the GNU/Linux cathedral what with its constant meddling, and as I've worked with them before, I don't have any desire to be part of the parade for which they're the drum majors.
Note I'm not criticising people here, but merely explaining my rationale for not using those.