Let me start this by saying I am not an Apple hater. Just to give my Apple cred, I have a Mac Mini, an iphone 4s and 5 and an original iPad.
I still run a Windows desktop though.
Let me explain...
My feelings towards Apple have morphed and changed over the years, along with technologies and the fortunes of those that compete with them. I think both sides of the PC/Apple argument miss the point most of the time. Apple touts how secure its systems are vs Windows, to which I say "of course". If you are a hacker, are you going to spend your time working on the systems that cover 80-90% of the market, or one that covers 10-15%. The choice is obvious. It isn't that OSX is inherently any more secure, it just has fewer guns pointed at it at any given time.
This can be seen by the fact that now they are starting to make some market headway, trojans and viruses are starting to pop up at what Mac users would consider an alarming rate. More market share equals more scrutiny. Get used to it.
Most of the other arguments are similarly pointless. There are some differences that count though.
First off Apple is, at its core, a hardware company. The operating systems support the hardware. This is very evident in some of their choices. For instance, scalable fonts on the desktop.
I have used Windows, OSX, Ubuntu, CentOS, and several other operating systems. Every system but OSX allows you to change the size of the fonts on the desktop relatively easily, and it handles it well resizing elements to compensate. The only way to resize the fonts on the desktop of OSX is to use a hack, and it is kludgy, ugly and basically unusable. As the fonts enlarge the header bars don't, and pretty soon it just becomes an unreadable mess.
The practical upshot of this is as you go to these beautiful retina displays that Apple is putting out, it is impossible to use them at native resolutions and you have to reduce the resolution of the OS anyway. I use my Mac Mini as a media server, and even at 1080p on my 56" big screen TV, I have to sit 4 feet away from the screen to be able to read the fonts when I am in the OS. Seriously? In Windows I would blow up the text enough to be readable and keep the icons at the same size, and voilla! All is swell.
At some point in the development of OSX is was decided that the font size would be a fixed number of pixels. In my opinion this was just to simplify programming and get the product out. Make it big enough that it is readable on what was then the highest resolution screens and leave it at that. If they have a higher resolution screen that must mean they have a bigger screen, and thus it won't matter.
Except it does. Every monitor has its own resolution. Now with the "retina" displays coming out, if you dare to run OSX at the native resolution (as I understand it actually takes a hack to do it) the text gets so small that you can't do anything.
The OS was designed for their hardware, and if it didn't work with other stuff, tough. But now that they have changed their direction on hardware and gone to these ridiculously high resolutions, the old way isn't cutting it. Now everyone is going to have to wait for the OS to catch up to the hardware.
And that is the difference.
Windows and Linux variants need to anticipate a variety of environments, because they can be used on a virtually infinite number of configurations. OSX is designed to only run on a limited number of configurations, even if you could theoretically use it in others. Thus, they don't need to make the OS as flexible and can lock things down that shouldn't necessarily be locked down such as font proportions and size.
This is why they will ship out the iPhone 4s with a beta Siri, or the iPhone 5 with a flawed and barely beta maps program. The hardware and design rule, the software follows.
Is this bad?
Because Apple has such control over the whole ecosystem, their hardware is spectacular. Seriously, run an iPhone 5 through your fingers. It is an unbelievable accomplishment. In every way it is a design icon that will stand the test of time. It is all you can do not lick it. It pushes all the right buttons. But for all the detail in the hardware, things were left hanging on the OS side.
Apple is a hardware company first, with the OS and software following the lead of the hardware.
That is why there are so many people who run Windows on Macs. You get the best hardware, and a great OS. You pay through the nose to do it, but you get the best of everything.