This article is ever-so-slightly deprecated. Please see: http://profectium.blogspot.com/2012/08/comparison-of-window-managers-and.html
As you saw in my post below I do not use Unity. I use the currently unstable version of Gnome 3 on my Ubuntu operating system. Today's post is about comparing some popular desktop environments for Linux. XFCE / tiling fans, I'm sorry, but I am not included you in this debate. I do not have any experience with XFCE or many tiling window managers and I wouldn't be able to give a fair report on them.
What desktop environment you choose is a matter of preferences. From the four I have used they all did the same thing in a different, sometimes prettier way.
Some good points to make about this desktop environment is the familiarity Windows 7 users will have with be able to keep launchers for their favorite apps. This is something that old Gnome versions couldn't do prettily. It also reduces the amount of distraction and increases the screen size for maximum viewing convenience.
This paragraph used to be about the instability of gnome when trying to install it on Ubuntu 11.04, and yes, that is a very bad idea if you do not want to kill Unity. It may even make both desktop environments flawed if something goes terribly wrong. However, Ubuntu 11.10 is out and it has fixed the compatibility issues of having Gnome and Unity installed at the same time. In addition to fixing the problem, Gnome 3 has been added to the Ubuntu repositories, so installing it is as simple as using the software center.
I will admit right off the bat that I have little good to say about E17. It's selling point is the lightweight beauty that it has. It is true that for someone willing to make their DE a hobby that E17 is the best out their for only 4 Megabytes. Some things I noticed was the bottom bar was confusing to use at times and was terrible at hiding itself. It would always overlap any window that entered it's territory. Maximizing and minimizing buttons were included but completely broken. Right click doesn't work on the desktop because left-click takes it's place. It comes with no detection of previously edited global shortcuts, which I didn't really expect it too.
The only good parts about E17 is that is has the potential to be great AND it is incredibly lightweight (4MB). It comes as sort of a template. But for people not willing to spend days tuning it to their liking it is just not for them.
Kde 4.6 - Plasma Workspace
The release of KDE 4 attempted to change the style of the DE into a prettified masterpiece. One of the first things I discovered was the built-in desktop settings manager. Straight out of the box I was able to activate my wobbly windows, change my color schemes around, and add widgets without having to leave my desktop. Windows users would find it easy to use as well because of the K menu, which resembles the start globe from Windows Vista and Windows 7. There are little things thrown into this desktop environment that make it what it is. Things so small most of the time they go unnoticed. For instance, when a window closes it doesn't abruptly shut down. It sinks into a center point and fades away. Or when a popup from the side of the screen comes out it will glide back to where it came from.
KDE's desktop effects can be turned off with the command alt+shift+f12 if you have a bad/old graphics card. Upon installation it checks to see if your settings allow you to run the Plasma workspace, so it may already be off by default.
The only bad things to say about this desktop environment are that it is not lightweight and that Gnome applications (If you install it over Gnome) keep themes. The download of Kubuntu(Ubuntu with KDE instead of Unity by default) is around 140MB if you already have the kernel and Libreoffice(It tries to install the Libreoffice suite on installation).
Kubuntu will also install KDE-related software if you don't deselect it. (Don't unless you know what you are doing. The Window manager is NOT enough to run KDE). This could lead to jumbled app lists like a System Settings Clone or Kate and Gedit. Two programs that do the same thing but each designed for another DE. Any program from Gnome works on KDE and vice versa, though. Like I mentioned earlier only KDE applications retain themes in KDE.
Extra: KDE can be installed on Windows and Mac too.
KDE also features a Sea of Windows, but it needs work.
Originally this paragraph hailed KDE as the winner of my competition, but now I feel that Gnome 3.2 has blown it out of the water. This is just opinion, since all of them are very usable desktop. There are the occasional crashes with Gnome, but it's simple, efficient when you start learning the ins and outs of the system (like wikipedia querys from the application search bar), and customizable through CSS. The height of the interference to the user is the occasional popup of the process dock and the always-present activities bar.
To download Gnome 3 your distribution probably has it in the repositories. If not and you would like to try it, there are other versions of Linux that were built with Gnome 3 in mind, like Fedora 16.