I've been using Linux for a long time, and I learned to use it by installing it and using it as my primary operating system — as far as I'm concerned, the year of the Linux desktop was in 1994 or so. Since then I've actually bounced around to a variety of operating systems and even single-booted my primary machine into Windows XP by choice, using Cygwin as a crutch. I've been running Linux solidly since Ubuntu Dapper, using Windows only on inferior (and seldom-activated) computers or in virtual machines to close corner cases like my mobile phone. Throughout that time, I have basically never been satisfied with the default appearance of the system.
When I started with Linux, Slackware was on version 2 and I installed (part of) it from a small stack of floppies. My full-AT 386DX25 with 8MB of DIP DRAM on the motherboard and 120MB of ATA disk was just capable of supporting the A, N, and D sets as well as Netscape. I didn't have much disk space left to actually do anything, but I was sitting at a fully-capable system... and I just had to customize it, which back then meant afterstep. Later on, the distributions changed, and so did the eye candy; Enlightenment was the vogue for a long time, for example, with its retro-futuristic themes and baroque (and unreadable) fonts.
Today the current state-of-the-art in computer desktops involves 3D acceleration. While there are next-generation projects like CAVE interfaces or even relatively simple rotational desktops like Metisse, in general this effort is focused on eye candy. This sort of functionality was very much accelerated in Linux by its appearance in Mac OS X, which is capable of accelerating every windowing operation through the GPU. Windows Vista also added this functionality to Microsoft's Windows NT operating system. Clearly, Linux needed to trump them both. OS X and Vista also added "desktop applet" functionality; there are actually several systems for this on Linux, but honestly no one seems to have the rich selection of applets presented on OS X or by Google or Yahoo on Windows (as opposed to by Microsoft on Windows.)
Finally, OS X blew away the world with eye candy in the form of the Dock, a somewhat less-usable version of the NeXTStep Dock. This pretty, translucent, amazing expanding-and-shrinking travesty of UI guidelines
While OS X is easily the smoothest example of an accelerated desktop, it is also the least configurable. This is typical of Apple's offerings; it's their way, or the highway. Windows XP is somewhat more configurable, but lacks every feature of the modern accelerated desktop except support for transparency, which is built into the OS but management of which requires the use of a third party utility and even there you must patch an OS DLL to permit the use of third-party themes (Microsoft has provided only a couple themes, plus some color schemes.) Linux, on the other hand, is extremely themable. Whether you use a KDE, Gnome, or even Xfce desktop, it is possible to theme your applications in hundreds of different ways, even to the extent that there are different theme engines which provide different theming capabilities. Some are even based on scalable SVG images, which permits them to look smooth at any resolution - providing a truly device-independent interface.
One interesting aspect of the Linux desktop is that it is possible to make it look and act like almost anything. GNOME has a very Unix-like behavior in general, and in many ways it behaves like CDE. There is a similar session-manager, for example, and while the default GNOME desktop now uses a menu bar-like panel at the top of the screen and a Windows-esque panel at the bottom with a task manager in a sort of Mac/Windows ambivalence, you can change it to anything you like. By using the gnome session manager, I disabled the panel and enabled avant-window-navigator, removing the traditional GNOME panels entirely and using a Mac-like dock. The dock has several system menus to choose from, so I can launch any program (not just those which have launchers on the panel) and really need no other menu, task switcher, et cetera. There are various dock applets of course, so I have a CPU meter and so on.
In order to really provide all the beautiful functionality of the OSX desktop, you really need to use compiz. On Ubuntu linux, this is referred to simply as "Desktop Effects". Turning this on via the "Appearance" preferences allows you to enable functionality like Expo, which lets you zoom out to see all your virtual desktops at once and drag applications between them. While there is the completely eye-candy rotatable "Desktop Cube", there is also a "Desktop Wall" plugin which provides a flat array of virtual desktops which goes well with Expo, and which permits simply dragging an application to the next desktop. Compiz Fusion also provides a Widget Layer plugin which goes nicely with any Desktop Widget system - in my case, since I am using the GNOME desktop, I chose gDesklets. This system drops the saturation (or other properties) of non-widget windows and shows your widgets when you activate via mouse, keyboard, or screen corner/edge; clicking away or executing the activation function again hides them, much like the functionality available in OS X.
Finally, in order to truly experience the accelerated linux desktop, you need to install emerald. By default, compiz on Ubuntu uses a GTK+ decorator, thus drawing your windows the same way that the default GNOME window manager "Metacity" would. If you want access to niceties like transparent window borders, emerald is the answer; it provides a somewhat fancier decorator with nifty theme engines like "truglass" which can be used to make extremely attractive and fully-alpha-blended window borders.
Now, the bad parts: if you have anything other than a highly-supported nVidia card, one or more of the pieces of this puzzle can more or less be counted on to break. On Ubuntu Gutsy, there is a common bug causing white screens if xserver-xgl is installed; My nVidia Quadro FX1500 has display corruption around the mouse pointer while dragging windows any time desktop effects are enabled. nVidia drivers and the Bicubic texture filter have problems interacting. ATI users are, as usual, in deep trouble most of the time. Intel graphics users will just have a really slow desktop compared to anyone else.
I plan to write up some formal howtos down the road here, so this is really just a reminder to produce those articles later.