LXDE is a lightweight classical desktop -- and saying more beyond that is almost impossible. Its innovations, such as icons to minimize or shade all open windows, are convenient, but decidedly minor. And LXDE lacks a broad ecosystem of apps compared to older environments like GNOME or KDE. Those who want a no-nonsense, reliable desktop they don't have to think about should find it just what they are looking for.
Those who miss GNOME 2 need look no further than MATE. Originally a fork of GNOME 2, MATE remains dedicated to preserving GNOME 2's look and performance. Features may have different names -- for instance, the Nautilus file manager becomes Caja -- but the functionality is virtually identical with the later GNOME 2 releases.
Not that MATE is a fossil, dedicated to resisting change. Rather, it advertises itself as a "continuation" of GNOME 2. It replaces GNOME's classical menu with a windowed menu, and functions such as software installation and updates are constantly being tweaked. However, generally changes are conservative.
If you have used GNOME 2 any time in the last ten years, then MATE should be a comfortable environment for you. In fact, even those for whom MATE is not their first choice are likely to make it their second choice.
Ubuntu's founder Mark Shuttleworth challenged the free software world to produce desktops that rivaled the OS X. His response to his own challenge was Unity, which was developed largely between 2010-12, and continues to be tweaked.
Unity's original design involved a radical simplification of the desktop that eliminated all panel apps except a few indicators, and app icons on the desktop. Further simplification is achieved by making application menus invisible until the cursor moves to them, and by replacing the classic menu with the dash, a display that fills the entire desktop. The taskbar to show open applications was replaced by a discreet set of triangle indicators, and applications open maximized on the desktop.
New Unity users are likely to be puzzled until they explore a bit, clicking and paying attention to mouse-over help. Yet, even so, Unity is probably the simplest of the desktops mentioned here. Casual users who only open one or two applications at a time are likely to find it suitable to their needs but more demanding users are likely to find it too restrictive, and, at times, dedicated to doing things differently simply for the sake of being different.
Note that recent Unity releases have added Internet search in the dash, a move that was both unrequested and contrary to the simplicity that is one of Unity's main design principles. Aside from the privacy issues raised by these Internet searches, their main result is to make finding items on your hard drive unnecessarily difficult. Turning this feature off is likely to make Unity more acceptable -- but even so, many users feel that Unity restrains both their work flow and their customizations so much as to be an annoyance.
Much of the recent work on Unity has involved tweaking it so that Ubuntu users can have much the same desktop regardless of whether they are working on a workstation or a tablet. Assuming that this promise is realized, it may be the main reason for choosing Unity in the future.
For years, Xfce was the third desktop of free software, trailing far behind GNOME and KDE. Today, however, reader polls consistently put it in second behind KDE and well ahead of GNOME.
It's the users, not Xfce, who have changed. Xfce's home page has declared its goals for years: "to be fast and low on system resources, while still being visually appealing and user friendly." This balance of goals has made Xfce a favorite of organizations like Free Geek and Reglue that refurbish aging computers and distribute them to the needy
However, while Xfce accomplishes both its goals extremely well, they are only part of its appeal. Its Thunar file manager with its library of extensions has a cult following, and Xfce frequently runs KDE and GNOME apps better than KDE or GNOME run each other's. If you run mixed applications, Xfce might be exactly the stable base for your system that you've been looking for.
The Diversity to Give You What You Want
Looking at this list, some readers are sure to suggest that having all these desktops are wasteful, that free software would be in a much better position if it only had a single desktop, like Windows or OS X.
They may have a point, so far as efficiency goes. But, from another point of view, the selection is a tribute to diversity and to giving users what they want.
This diversity can be confusing, but fortunately none of the major desktop environments are inarticulate about the ideas behind their design. In fact, all of them are very thorough about carrying out their intents.
Once you know what each desktop environment is trying to accomplish, you should be able to decide which ones are for you without taking the trouble of booting each in a Live DVD. You may still want to try out a few, but at the very least, you can simplify your selection. Given the selection, you should be able to find something that fits your work habits and personal preferences.